^^/)^-^c>,-vw^;^^^^- 


f. 


^-/"^ 


REPORT 


ON  THE 


■lACKWELL'S   ISLAND  BRIDGE 

(QUEENSBORO  BRIDGE) 


C.  KUNZ 

Chief  Engineer 


THE   PENNSYLVANIA   STEEL   CO, 


fernia 
laJ 


And  a  Commission  consisting  of 
CHARLES    MACDONALD,  Consulting   Engineer 

Past-President  Am.  Soc.  C.  E. 

C.    C.    SCHNEIDER,  Consulting   Engineer 

Past- President  Am.  Soc.  C.  E. 

H.    R.    LEONARD,  Consulting   Engineer 
J.    E,    GREINER,   Consulting   Engineer 


Steelton,  Penna.y  March  24,  1909 


REPORT 


ON    THE 


BLACKWELL'S    ISLAND   BRIDGE 

(QUEENSBORO   BRIDGE) 


By   F.   C.   KUNZ 

Chief  Engineer 

THE    PENNSYLVANIA    STEEL   CO. 


And  a  Commission  consisting  of 
CHARLES    MACDONALD,   Consulting    Engineer 

Past-President  Am.  Soc.  C.  E. 

C.    C.    SCHNEIDER,   Consulting    Engineer 

Past-President  Am.  Soc.  C.  E. 

H.    R.    LEONARD,   Consulting   Engineer 
J.    E.     GREINER,   Consulting    Engineer 


Steelton,  Penna.,  March  24,  1909 


UlSount  Ipleasiant  f^ttii 

J.  Horace  McFarland  Company 
Harrisburg,  Pennsylvania 


5RLF      OtI    SS&^^^'^O 


URL 


INDEX 


PAGE 


General  Elevation  and  Progress  Photograph  of  Bridge ..  Frontispiece  / 
Queen's  Approach,  View  Looking  through  Bridge  Showing'Main  Road- 

wa}^ Frontispiece  II 

Letter  Transmitting  Chief  Engineer's  Report  to  Commission 5 

Report  of  Commission   7-13 

Chief  Engineer's  Report    15-46 

Supplement  to  Chief  Engineer's  Report 47 

Appendix  A:    Extracts  from  Reports  of  Experts  49-51 

Appendix  B:  Extracts  of  Specifications  for  the  Steel  Superstructure  52-57 
Cross  Section  of  Bridge  with  Two  Rapid  Transit  Railroad  Tracks...  58 
Cross  Section  of  Bridge  with  Four  Rapid  Transit  Railroad  Tracks. .  .        59 

Photograph  of  Crowd  ^in  Coney  Island    61 

Photograph  of  Crowd  on  Ferryboat 63 

Photograph  of  Gathering  of  Teams  65 

Table  1:    Diagram  of  "Continuous"  Live  Load  for  Chords. 
Table  2:    Diagram  of  "Continuous"  Live  Load  for  Web  Members. 
Table  3:    Diagram  of  "Discontinuous"  Live  Load  for  Chords. 
Table  4:    Diagram  of  "Discontinuous"  Live  Load   for  Web  Members. 
Table  5:    Unit  Stresses  for  Various  Conditions  of  Loading  with   Origi- 
nal  Paving. 
Table  6:    Unit  Stresses  for  Various  Conditions  of  Loading  with  Final 

Paving. 
Table  7:    Diagram  Showing  Congested   Loading. 


BlackweU's  Island  (Queensboro)  Bridge 


THE   PENNSYLVANIA   STEEL   COMPANY 

OFFICE    OF   THE   VICE-PRESIDENT 


J.   V. 


W.  Retnders 

Vice-President 


Steelton,  Pa.,  December  22d,  1908. 
Mr.  Charles  MacDonald, 
Mr.  C.  C.  Schneider, 
Mr.  H.  R.  Leonard, 
Mr.  J.  E.  Greiner. 

Dear  Sirs:  The  Pennsylvania  Steel  Company,  on  November  20th,  1903, 
contracted  with  the  City  of  New  York  to  furnish  the  steel  superstructure 
of  the  Blackwell's  Island  Bridge,  in  accordance  with  plans  and  specifications 
prepared  by  the  Department  of  Bridges,  under  the  commissionership  of 
Mr.  Gustav  Lindenthal.  On  December  15th,  1904,  the  city  entered  into 
a  supplementary  contract  with  the  Steel  Company,  providing  for  certain 
work  not  originally  contemplated,  including  the  addition  of  two  elevated 
railroad  tracks.  The  work  was  completed  June  15th,  1908,  and  a  certificate 
of  acceptance  issued  by  the  Department  of  Bridges. 

In  the  spring  of  1908  articles  appeared  in  one  of  the  New  York  daily 
papers  criticizing  the  design  of  the  bridge  and  drawing  analogies  between 
it  and  the  Quebec  Bridge,  which  collapsed  some  months  previous.  Sub- 
sequent investigations  of  the  structure,  conducted  at  the  instance  of  the 
Department  of  Finance  and  the  Department  of  Bridges,  of  the  City  of  New 
York,  by  Prof.  William  H.  Burr  and  Messrs.  Boiler  &  Hodge,  have  led  to  many 
serious  misconceptions  in  the  public  mind.  Proper  appreciation  of  the  find- 
ings of  these  engineers  presupposes  a  clear  understanding  of  the  original 
assumptions  upon  which  the  specifications  and  general  plans  were  drawn, 
and  the  significance  of  which  must  be  read  into  the  computations  which 
formed  the  basis  of  the  reports. 

As  far  as  that  part  of  the  work  is  concerned  for  which  the  steel  contractor 
was  responsible,  the  reports  are  uniformly  favorable,  the  following  con- 
clusions of  Messrs.  Boiler  &  Hodge  being  characteristic  of  both  reports,  viz.: 


"(Second)  That  the  steel  manufactured  for  this  structure  is  first-  Conclusions  i 

class  bridge  material  and  in  accordance  with  the  specifications.  Boiler  &  '  ' 

"(Third)  That    the    workmanship    of   this    structure   is    first-class  Hodge  ; 

and  in  accordance  with  the  requirements  of  the  specifications.  i 

"(Fourth)  That  the  erection   and  field  riveting  of  the  structure  \ 

appears  to  have  been  done  in  a  first-class  manner.  j 

"(Fifth)  That  the  actual  sections  of  the  various  members  agree  | 


6         LETTER  TRANSMITTING  CHIEF   ENGINEER'S  REPORT 

with  the  sections  ordered  on  the  working  drawings  and  shown  on 
our  sheets  Nos.  8  and  9,  and  that  the  shipping  weights  are  correct." 

(See  Appendix  A,  page  49,  for  other  extracts.) 

While,  therefore,  from  the  point  of  view  of  a  contractor  we  are  not  in- 
volved in  any  of  the  issues  that  have  been  raised,  it  is  proper  that  our  knowl- 
edge of  the  situation  should  be  made  available  both  for  the  information  of 
the  engineering  profession  as  well  as  the  general  public,  whose  sense  of  security 
in  respect  to  this  structure  has  been  unduly  disturbed. 

From  this  point  of  view,  we  have  asked  our  Chief  Engineer,  Mr.  F.  C. 
Kunz,  to  prepare  a  report  setting  forth  all  the  salient  points  that  have  been 
raised  from  time  to  time,  with  such  information  as  we  are  able  to  supply 
in  regard  to  the  same,  and  we  now  ask  that  you,  as  a  Commission,  carefully 
examine  this  report,  advising  us  whether  you  agree  or  disagree  with  the 
conclusions  as  set  forth  therein,  and  stating  briefly  the  grounds  upon  which 
your  opmion   is   based. 

Very  truly  yours, 

J.  V.  W.  REYNDERS, 

Vice-President. 


Report  of  Commission 


The   Pennsylvania   Steel'^^Company, 

J.  V.  W.  Reynders,  Vice-President,  Steelton,  Pa. 

Dear  Sir:  Since  the  failure^of  the  Quebec  Bridge,  public  confidence  has 
been  somewhat  disturbed  as  regards  the  safety  of  bridges  of  unusual  mag- 
nitude. This  feeling  of  distrust  has  been  aggravated  by  the  opinion  expressed 
in  the  report  of  the  Royal  Commission,  appointed  to  inquire  into  and  report 
on  the  cause  of  the  failure  of  the  Quebec  Bridge;  this  report  was  published 
and  has  been  extensively  quoted  by  the  technical  journals  in  this  country 
as  well  as  abroad.  The  unwarranted  remark  contained  in  this  report,  that 
"under  extreme  conditions,  the  Quebec  Bridge  stresses  are  in  general  har- 
mony with  those  permitted  in  the  Black  well's  Island  Bridge,"  produced 
the  impression  in  the  minds  of  the  New  York  public  that  the  Blackw^ell's 
Island  Bridge  might,  sooner  or  later,  share  the  fate  of  the  Quebec  Bridge. 

For  the  purpose  of  obtaining  an  unbiased  opinion  as  to  the  true  con- 
dition of  the  Blackwell's  Island  Bridge,  the  Commissioner  of  the  Depart- 
ment of  Bridges  of  New  York  City  appointed  two  experts  to  examine  the 
design  and  construction  of  this  bridge. 

Owing  to  the  technical  nature  of  their  reports  and  a  lack  of  clear  under- 
standing of  the  significance  of  the  assumptions  upon  which  the  computations 
were  based,  the  statements  and  conclusions  contained  therein  have  led  to  many 
serious  misconceptions  in  the  public  mind.  They  have  been  misunderstood 
and  misinterpreted  by  engineers  who  are  not  experts  in  bridge  design,  have 
been  used  by  a  small  section  of  the  local  engineering  press  as  a  basis  for  the 
unjust  assumption  that  the  early  designers  of  the  bridge,  as  well  as  those 
who  followed,  blundered  seriously,  and  foreign  technical  journals  have  taken 
the  opportunity  for  the  abuse  and  wholesale  condemnation  of  American 
practice  in  general,  and  the  judgment  of  American  engineers  in  particular. 

The  pubUc  confidence,  which  was  disturbed  by  the  Quebec  failure  and 
by  the  unwarranted  comparison  of  that  bridge  with  the  Blackwell's  Island 
Bridge  structure,  has  certainly  not  been  restored  by  the  reports  of  the  city's 
experts  on  the  latter  bridge.  In  fact,  the  strength  of  the  Blackwell's  Island 
Bridge  has  now  become  a  question  of  such  serious  and  far-reaching  impor- 
tance, affecting  not  only  the  confidence  of  the  public  in  engineering  works, 
but  the  professional  standing  of  American  engineers,  that  it  is  assuredly 
proper  and  advisable  for  the  contractors  to  make  available  their  knowledge 
of  the  situation;  and  the  undersigned,  at  the  request  of  The  Pennsylvania 
Steel  Co.,  as  conveyed  in  your  letter  of  December  22nd,  1908,  have  reviewed 
the  report  of  your  Chief  Engineer  covering  this  subject. 

7 


8  REPORT   OF    COMMISSION 

We  have  carefully  examined  this  report,  dated  November  27th,  1908, 
together  with  all  data  and  information  furnished  in  connection  therewith, 
including  the  specifications,  contract,  strain  sheets,  reports  of  experts,  etc., 
etc.,  and  substantially  endorse  the  arguments,  conclusions  and  recommen- 
dations therein  set  forth. 

Our  report  should,  in  our  judgment,  be  an  answer  to  the  unjust  and 
disturbing  criticisms  that  have  appeared  in  the  public  prints  rather  than 
a  resume  of  the  entire  subject;  the  structure  has  been  so  overloaded  mathe- 
matically that  the  confidence  of  the  public  has  been  shaken,  and  it  is  only 
by  an  appeal  to  common  sense,  rather  than  to  technicalities,  that  this  lost 
confidence  can  be  regained. 

No  question  has  been  raised  as  to  the  sincerity  of  the  contractors  in  exe- 
cuting the  work;  and  the  quality  of  the  material,  character  of  workman- 
ship and  adherence  to  approved  plans  have  been  endorsed  by  the  city's 
experts.  It  is  our  understanding,  after  careful  study,  that  the  contract  gave 
free  powers  to  the  Bridge  Department  in  general  design  and  engineering 
changes,  and  does  not  presuppose  engineering  knowledge  on  the  part  of 
the  contractor  further  than  is  necessary  for  the  proper  execution  in  detail 
of  the  general  orders  of  the  Department. 

The  entire  argument  can  well  be  based  on  the  consideration  of  this  ques- 
tion: Were  the  original  specifications,  and  the  subsequent  modifications  made 
by  the  Bridge  Department,  of  such  a  nature  that  there  can  be  any  engineer- 
ing doubt  as  to  the  safety  and  usefulness  of  this  structure  as  a  public  high- 
way? 

A  bridge  is  a  highway  and  is  not  designed  primarili/  to  carry  so  many 
pounds  per  linear  foot,  but  to  accommodate  so  much  traffic,  and  from  an 
estimated  weight  of  such  traffic  the  design  is  perfected.  For  a  railroad  bridge 
such  weights  are  easily  determined,  but  for  a  roadway  structure,  the  deter- 
mination of  live  load  weights  is  largely  a  matter  of  good  judgment. 

No  material  difference  of  opinion  can  arise  as  to  the  proper  loading  to 
be  assumed  in  the  designing  of  minor  parts,  i.  e.,  hangers,  floor-beams,  stringers, 
etc.;  such  loading  would  cover  but  small  areas  and  a  maximum  density  of 
traffic  could  easily  be  conceived,  but  for  the  main  trusses  in  a  structure 
of  any  magnitude,  the  application  of  live  loads  of  maximum  density  ("con- 
gested") over  any  extended  areas  would  preclude  all  possibility  of  motion, 
a  condition  that  would  destroy  the  usefulness  of  the  structure  as  a  vehicle 
*of  traffic,  a  condition  so  absurd  that  it  would  not  be  tolerated  in  a  city  street. 

The  measure  of  usefulness  of  any  public  highway  is  the  amount  of  traffic 
that  can  be  safely  and  expeditiously  handled  over  same,  and  any  police 
regulation  made  for  the  proper  handling  of  such  traffic  is  a  necessity  for  the 
benefit  of  the  traveling  public  and  does  not,  in  itself,  lessen  confidence  in 
the   structure. 

Traffic  rules,  requiring  the  surface  cars  to  keep  to  certain  clear  distances 
apart  are  no  hardship;  quite  the  reverse,  in  allowing  for  greater  freedom 
of  motion,  they  are  in  the  interest  of  increased  traffic  and  consequently  in- 
creased capacity.    On  elevated  railroad  tracks,  proper  spacing  of  trains  is 


REPORT   OF   COMMISSION  9 

a  measure  of  safety,  and  the  greater  the  interval  of  space,  the  greater  could 
be  the  permissible  speed;  block  signal  and  automatic  devices  now  in  use 
absolutely  prevent  electric  trains  from  encroaching  on  the  space  deemed 
necessary  for  their  prompt  and  safe  handling. 

Live  Load.   Professor  Burr,  in  his  report  on  this  bridge,  states: 

"Proper  provision  for  various  classes  of  loading  for  a  structure 
of  such  magnitude,  designed  to  carry  an  extraordinary  volume  of 
traffic,  with  the  corresponding  working  stresses,  is  largely  a  matter 
of  judgment." 

In  this  connection,  we  wish  to  call  attention  to  the  investigations  of  a 
Commission  appointed  in  1903  by  the  Mayor  of  New  York  City  to  examine 
and  pass  upon  the  plans  of  the  Manhattan  Bridge,  which  was  then  designed 
as  a  suspension  bridge  with  stiffened  eye-bar  cables.  While  the  report  of 
this  Commission  refers  wholly  to  the  Manhattan  Bridge,  the  recommenda- 
tions as  to  the  live  loads  for  which  it  should  be  designed  have  been  applied 
to  the  Blackwell's  Island  structure,  which  was  intended  to  carry  the  same 
kind  of  traffic.  This  Commission,  consisting  of  Messrs.  George  S.  Morison, 
C.  C.  Schneider,  Henry  W.  Hodge,  Mansfield  Merriman  and  Theodore  Cooper, 
made  a  very  thorough  investigation  of  the  subject  and  recommended  that 
the  Manhattan  Bridge  should  be  designed,  in  so  far  as  the  main  members 
are  concerned,  to  carry  a  "maximum  working"  load  of  8,000  pounds  per 
Unear  foot,  and  that  a  so-called  "congested"  load  of  16,000  pounds  be  used 
in  proportioning  the  hangers.  Quoting  from  this  report,  in  regard  to  further 
use  of  the  "congested"  load, 

"We  consider  that  the  bridge  should  be  so  proportioned  that 
■  with  a  congested  load  of  16,000  pounds  per  Unear  foot,  covering  the 
whole  bridge,  combined  with  dead  load  and  wind  pressure,  no  stresses 
should  be  produced  anywhere  reaching  the  elastic  limit  of  the  material 
or  impairing  the  stability  of  the  anchorages.  We  consider  that  the 
working  load  of  8,000  pounds  per  linear  foot  should  be  used  in  design- 
ing the  main  members  of  the  structure.  The  congested  load  should 
be  used  in  proportioning  the  hangers." 

Any  fair  interpretation  of  this  report  would  indicate  that  the  intention 
of  the  Commission  was  to  recommend  the  use  of  the  "congested"  load  for 
proportioning  the  hangers  (using  a  working-unit  stress)  and  as  an  extreme 
test  of  the  cables  and  anchorages;  and  that  the  "working"  load  be  used  in 
proportioning  the  main  members  of  the  structure.  The  above  quotation 
appears  (in  part)  in  the  report  of  one  of  the  city's  experts,  but  the  omission 
of  the  words  "or  impairing  the  stability  of  the  anchorages"  and  the  omission 
of  the  last  two  sentences  quoted  above,  admits  of  an  entirely  different  con- 
struction being  placed  on  the  recommendation. 

Independent  investigations  which  we  have  made  confirm  the  estimate 
of  the  Manhattan  Bridge  Commission  as  well  as  that  of  the  city's  experts, 
fixing  the  maximum  load  on  any  extended  area  of  roadway  or  footwalk  at 


10  REPORT   OF   COMMISSION 

50  pounds  per  square  foot.  With  the  increased  weights  of  elevated  and 
surface  cars,  cited  in  the  experts'  reports,  we  would  obtain  as  a  "congested" 
load,  15,955  pounds  per  linear  foot,  made  up  as  follows: 

4  elevated  8-car  trains,  at  1,810  pounds 7,240  per  lineal  foot 

4  trolley  tracks,  at  1,460  pounds 5,840  per  lineal  foot 

35.5  feet  roadway,  at  50  pounds  per  square  foot 1,775  per  lineal  foot 

22  feet  footwalk,  at  50  pounds  per  square  foot 1,100  per  lineal  foot 

15,955 

Such  a  load,  with  roadway  and  footwalk  crowded,  trolley  cars  bumper  to 
bumper,  and  elevated  tracks  completely  covered,  is  an  impossibility  unless 
special  and  extraordinary  means  were  taken  to  produce  it,  and  the  term 
"congested"  appUed  to  such  a  loading  in  connection  with  the  computations 
of  stresses  in  main  members  is  misleading;  a  "theoretical  test  load"  or 
"extraordinary  load"  would  be  terms  more  applicable. 

A  maximum  working  load  is  much  more  complex  of  analysis  than  a  "con- 
gested" load,  and  is  a  matter  on  which  the  judgment  of  engineers  may  be 
expected  to  differ.  With  a  "congested"  load  provided  for,  the  possibility 
of  failure  from  collapse  is  eliminated,  but  to  provide  for  such  a  load  at  ordi- 
nary working  units  would  be  an  unwarranted  extravagance,  hence  the  econom- 
ical necessity  for  determining  a  working  load  and  providing  for  such  loading 
with  working  units.  The  working  load  of  8,000  pounds  per  linear  foot  of 
bridge  recommended  by  the  Manhattan  Bridge  Commission  and  used  in  the 
computations  of  the  Blackwell's  Island  structure  by  the  Bridge  Department 
could  be  analyzed  as  made  up  of  20  pounds  per  square  foot  on  the  footwalks 
(an  uncomfortable  walking  crowd),  30  pounds  per  square  foot  on  the  road- 
way (equivalent  to  a  semi-congestion  of  average  vehicles),  1,945  pounds 
per  linear  foot  on  four  trolley  tracks  (equal  to  heaviest  loaded  cars  spaced 
two  car-lengths  apart)  and  4,550  pounds  per  linear  foot  as  an  equivalent 
load  for  the  elevated  trains  on  four  tracks,  trains  spaced  about  1,000  feet 
apart;  such  loading  can  be  conceived,  but  it  is  doubtful  whether  these  weights 
would  be  reached  once  in  a  period  of  years. 

Dead  Load.  Among  the  first  criticisms  appearing  in  the  public  prints 
was  the  assertion  that  the  dead  load  had  been  enormously  increased  with- 
out a  corresponding  increase  in  size  of  the  main  carrying  members.  This 
assertion, — one  that  would  appeal  directly  to  the  fears  of  the  public,  for 
whose  benefit  this  bridge  has  been  constructed,  appeared  in  one  of  the  lead- 
ing daily  papers,  and  was  answered  at  the  time  by  the  Bridge  Department, 
but  it  can  be  better  answered  now  by  a  frank  discussion  of  the  entire  dead- 
weight problem. 

The  weight  of  steel  now  in  place,  plus  an  estimated  weight  of  the  addi- 
tional steel  required  to  complete  the  overhanging  footwalks,  amounts  to 
106,650,000  pounds.  The  same  items  were  assumed,  in  the  calculations 
of  1904,  at  103,100,000  pounds.  The  assumptions  as  to  these  items  were 
therefore  exceeded  but  3^  per  cent. 

The  uniform   loading   (other  than   steel)    covering  the   weight   of  track 


REPORT   OF   COMMISSION  11 

for  both  elevated  and  surface  cars,  hand-rails,  paving  for  roadway  and  foot- 
walks,  pipes,  etc.,  assunaed  in  the  1904  calculations,  was  5,109  pounds  per 
foot,  aggregating  19,030,000  pounds.  This  weight  was  estimated  from  such 
plans  of  the  structure  as  were  perfected  at  that  time;  but  the  Bridge  Depart- 
ment in  1907  changed  the  plans  as  to  roadway,  increasing  the  weight  of  pav- 
ing, inside  trolley  rails  and  hand-rails,  making  the  uniform  load  (other  than 
steel)  6,968  pounds  per  linear  foot,  or  an  increase  of  1,859  pounds  for  these 
items.  This  was  an  admitted  error,  and  has  been  partially  remedied  by  reduc- 
ing the  weight  of  paving,  etc.,  on  the  river  spans  to  the  extent  of  1,168  pounds 
per  foot,  no  change  being  made  on  the  anchor  and  island  spans,  as  the  added 
dead  weight  on  these  spans  reheved  the  maximum  stresses  to  some  extent. 
The  total  dead  weight  of  the  structure,  with  four  elevated  tracks  and 
overhanging  footwalks,  would  then  be  about  130,700,000  pounds  against 
an  assumed  (1904)  weight  of  about  122,130,000  pounds,— an  increase  of  6^ 
per  cent.  The  dead  load  stre.sses,  however,  are  not  increased  to  the  above 
extent,  as  the  distribution  of  increased  weights  is  not  uniform,  a  large  pro- 
portion of  increase  in  steel  having  gone  to  the  towers  and  anchorages. 

The  design  of  this  structure,  a  cantilever  bridge  without  a  suspended 
connecting  span,  gives  a  continuity  not  found  in  the  ordinary  cantilever, 
inasmuch  as  a  load  on  any  part  of  the  bridge  affects  the  stresses  in  the  entire 
structure  from  end  to  end.  A  strict  interpretation  of  the  specifications  requires 
the  loads  to  be  placed  in  such  positions  as  to  give  the  greatest  stress  on  any 
member  of  the  structure.  The  Bridge  Department,  in  preparing  the  strain 
sheets  from  which  the  bridge  was  built,  did  not  strictly  follow  this  clause, 
and  applied  the  loads  both  "working"  and  "congested"  in  one  continuous 
stretch,  this  stretch  of  any  length  covering  one  or  more  of  the  subdivisions 
of  the  bridge  or  the  entire  length  of  the  structure,  but  with  no  unloaded 
gaps.  The  city's  experts,  in  making  their  analysis  of  the  structure,  inter- 
preted the  specifications  literally,  and  obtained  stresses  (the  mathematical 
accuracy  of  which  we  do  not  question)  alarmingly  high,  when  considering 
the  "congested"  load  together  with  the  increased  weight  of  pavement. 

A  reasonable  and  proper  distribution  of  assumed  live  load  on  a  struc- 
ture of  this  type  and  magnitude  is  again  a  matter  of  engineering  judgment. 
The  adopted  method  of  the  Bridge  Department  was  well  within  their 
rights,  especially  as  regards  the  so-called  "congested"  load,  and  it  is  our 
judgment  that  such  placing  of  the  loads  would  cover  all  possible  contin- 
gencies liable  to  arise. 

The  alarmingly  high  stresses  obtained  by  the  experts,  as  stated  before, 
were  arrived  at  by  a  strict  interpretation  of  the  printed  specifications  by 
placing  the  "congested"  load  of  16,000  pounds  per  linear  foot  on  certain 
fixed  portions  of  the  bridge  with  fixed  lengths  of  gaps  in  which  there  could 
be  no  load  whatever;  a  method  that  might  well  be  described  as  the  placing 
of  impossible  loads  in  an  impossible  manner. 

Professor  Burr's  method  of  calculation  of  stresses  produced  by  the  ele- 
vated railroad  trains,  spacing  eight  car  trains  in  position  to  give  maximum 
stress,  but  not  less  than  1,000  feet  apart,  is  rational,  and  we  fully  endorse 


12  REPORT   OF   COMMISSION       . 

his  method;  but,  ■T;vhen  more  than  one  track  is  treated  in  this  manner,  some 
concession  should  be  made  either  in  unit  stresses  or  weight  of  trains  for 
economic  reasons;  the  possibihty  that  two  or  more  trains  of  410  feet  length 
(eight  cars)  fully  loaded,  occupying  exact  spaces  on  one  track,  should  be 
provided  for;  that  this  same  loading  and  spacing  could  occur  on  a  second 
track  at  the  same  instant  of  time  is  only  conceivable,  but  that  all  four  tracks 
should  be  loaded  in  this  exact  manner  is  well  nigh  impossible,  and  places 
such  loading  immediately  in  the  category  of  "congested"  loads  to  be  pro- 
vided for  by  a  higher  unit. 

According  to  Professor  Burr's  conclusions,  "a  controlled  traffic  on  the 
four  trolley  lines  of  the  lower  deck  and  on  two  elevated  railways  of  the 
upper  deck,  carrying  the  heaviest  cars  of  their  classes  now  in  use  in  the 
City  of  New  York,  together  with  a  vehicular  traffiic  on  the  roadway,  and 
two  loaded  sidewalks,  may  be  permitted  without  exceeding  the  specified 
unit  stresses  for  the  regular  live  load  and  dead  load  and  without  exceeding  the 
safe  limits  of  stresses  for  such  a  structure."  This  conclusion  was  based  on 
reducing  the  dead  load  by  a  considerable  amount. 

We  have  made  an  investigation,  using  Professor  Burr's  method  of  dis- 
tribution of  train  load  on  four  elevated  tracks,  together  with  8,000  pounds 
per  linear  foot  of  bridge,  and  find  that  the  stresses  produced  by  this  extreme 
load  practically  agree  with  those  specified  for  the  congested  load,  and  are, 
therefore,  well  within  the  limits  of  safety. 

Considering  the  character  of  the  structure  and  assumed  loads,  the  unit 
stresses  specified  and  used  in  the  computations  were  conservative;  a  dis- 
tinction should  be  made,  how^ever,  between  unit  stresses  for  "working" 
loads  and  "congested"  loads.  The  City's  experts  recommend,  with  one  excep- 
tion, the  unit  stresses  for  working  loads  fixed  by  the  specifications,  the  excep- 
tion being  a  slightly  higher  unit  for  steel  in  compression,  due  to  change  in 
reduction  formula.  One  of  the  experts,  after  listing  "working"  unit  stresses 
substantially  in  accord  with  the  specifications,  stated  that  these  stresses 
"are  the  limit  of  safety  for  the  direct  stresses  from  the  sum  of  the  live  and 
dead  loads,  as  the  secondary  and  snow  load  stresses  heretofore  referred 
to  will  add  to  these  stresses."  The  secondary  stresses  are  small,  especially 
in  the  tension  members,  where  the  higher  units  are  specified,  and  we  believe 
that  a  snow  load  may  be  safely  neglected  when  considering  working  or  "con- 
gested" loads;  it  would,  therefore,  seem  that  the  term  "Umit  of  safety," 
as  applied  to  such  working  stresses,  was  unfortunate  and  tending  to  cause 
unnecessary  alarm.  The  "limit  of  safety"  would,  in  a  theoretically  perfect 
structure,  be  just  under  the  elastic  limit  of  the  material;  secondary  stresses 
and  imperfect  distribution  of  stresses  should  be  allowed  for,  and  we  believe 
that  sufficient  allowance  was  made  for  such  factors  in  the  specifications,  in 
fixing  on  the  unit  stresses  to  be  used  in  connection  with  the  "congested" 
load. 


REPORT    OF   COMMISSION  ,  13 


CONCLUSIONS 


(1)  We  are  of  the  opinion  that  the  live  loads  provided  for  in  the  original 
specifications,  with  the  subsequent  modifications  made  by  the  Bridge  Depart- 
ment, both  as  to  weights  and  distribution  of  same,  are  sufl&cient  for  the  traffic 
the  bridge  is  intended  to  carry,  and  cover  all  possible  contingencies. 

(2)  That  the  unit  working  stresses  specified  are  in  accordance  with  good 
practice,  and  the  limiting  stresses  for  extreme  conditions  of  loading  are  well 
within  safe  limits. 

(3)  That  the  actual  weight  of  steel  superstructure  practically  agrees  with 
the  estimated  weight  used  in  calculating  the  stresses,  within  the  usual  allow- 
ance permitted  in  bridge  work.  ^ 

(4)  That  the  superstructure,  as  built,  conforms  to  the  specifications  and 
designs  approved  by  the  Bridge  Department. 

(5)  That  the  bridge,  as  now  constructed,  with  provision  for  two  elevated 
tracks  is  entirely  safe  to  carry  all  traffic  which  can  possibly  come  upon  it 
under  present  conditions,  without  any  other  restrictions  than  those  necessary  to 
regulate  such  traffic  (see  cut,  page  58). 

(6)  That,  for  conditions  of  traffic,  i.  e.,  the  weight  of  vehicles,  surface  and 
elevated  cars,  as  now  existing,  the  bridge  would  also  be  safe  to  carry  all  the 
lines  of  traffic  contemplated  in  the  final  design  of  the  bridge  subject  to  ordinary 
traffic  regulations  (see  cut,  page  59). 

Respectfully  submitted, 

Charles  Macdonald, 
C.  C.  Schneider, 
H.  R.  Leonard, 

J.  E.  Greiner. 

March  8th,  1909 


Report  on  Blackwell's  Island  Bridge 


THE  PENNSYLVANIA  STEEL  COMPANY 

BRIDGE    AND    CONSTRUCTION    DEPARTMENT 
Thomas  Earle 

Superintendent 
F.  C.  KuNZ 

Chief  Engineer 

Steelton,  Pa.,  November  27th,  1908 

Mr.  J.  V.  W.  Reynders,  Vice-President. 

Dear  Sir:  The  Reports  of  the  Consulting  Engineers,  Prof."  William  H, 
Burr  and  Messrs.  Boiler  &  Hodge,  appointed  June  9th  by  the  Commissioner 
of  the  Department  of  Bridges  of  the  City  of  New  York,  in  accordance  with 
the  resolution  of  the  Board  of  Estimate  and  Apportionment,  dated  June  5th, 
to  examine  "the  design  and  structure"  of  the  Queensboro  Bridge,  formerly 
called  Blackwell's  Island  Bridge,  are  now  pubUc  property.  Coming  from  such 
an  eminent  source,  the  conclusions  of  the  reports  have  been  accepted  without 
question,  and  much  unfavorable  criticism  of  the  design  of  the  bridge  followed. 
It  would,  no  doubt,  interest  the  profession  to  know  that  there  are  engineers, 
not  responsible  for  the  design,  who  do  not  agree  with  all  the  conclusions  of 
the  reports.  It  would  seem  that  the  experts  were  not  in  possession  of  all  data 
concerning  the  design,-  which  would  not  be  surprising,  considering  the  fact 
that  the  construction  of  this  bridge  was  directed  by  three  different  adminis- 
trations of  the  Department  of  Bridges.  Furthermore,  while  the  reports  are 
fair,  they  are  based  on  assumptions  different  from  those  originally  made. 

Professor  Burr  remarks  at  the  beginning  of  his  criticism:  Permissible 

Unit  Stresses 

"Proper  provision  for  various  classes  of  loading  for  a  structure  of  Depend  on 
such  magnitude,  designed  to  carry  an  extraordinary  volume  of  traffic  Live  Load 
with  corresponding  working  stresses,  is  largely  a  matter  of  judgment." 

Here  lies  the  whole  difficulty.  After  the  necessary  assumptions  have  been 
made,  the  rest  is  a  mechanical  procedure,  and  the  bridge  will  be  declared 
"unsafe  in  some  members  and  wastefuUy  proportioned  in  others,"  or  "safe 
and  uniformly  proportioned,"  depending  on  these  assumptions;  the  moment 
they  are  changed,  the  verdict  may  be  totally  reversed.  It  is  safe  to  state 
that,  the  assumptions  being  a  matter  of  judgment,  in  other  words,  one  on  which 
engineers  may  honestly  hold  different  opinions,  no  two  engineers,  working 
independently,  would  be  likely  to  recommend  the  same  maximum  live  loads 
and  corresponding  unit  stresses  for  a  structure  of  the  magnitude  of  the  Black- 
well's  Island  Bridge,  with  its  two  independent  floors,  accommodating  four- 
teen lines  of  traffic  of  four  different  kinds,  viz.:  four  trolley  tracks,  four 
elevated  railroad    tracks,   four  rows    of  wagons   on   the   roadway  and   two 

15 


16  CHIEF   ENGINEER'S   REPORT 

promenades  for  pedestrians.  It  is  true  that  there  were  specifications;  but 
all  specifications,  even  for  ordinary  bridge  work,  can  be  interpreted  in  dif- 
ferent ways  by  different  engineers. 

The  original  specifications  for  the  Blackwell's  Island  Bridge,  written  in 
1903,  prescribed  a  "regular"  live  load  of  6,300  pounds  and  a  "congested" 
live  load  of  12,600  pounds  per  linear  foot  of  bridge,  which  in  April,  1904,  were 
changed  to  8,000  pounds  and  16,000  pounds  respectively,  when  the  Depart- 
ment of  Bridges  decided  to  add  two  elevated  railroad  tracks,  making  the 
traffic  facilities  equal  to  those  of  the  Manhattan  Bridge.  These  live  loads 
were  adopted  from  a  report  submitted  to  the  Department  of  Bridges  early  in 
1903,  by  a  commission  of  bridge  experts  appointed  to  examine  and  pass  upon 
a  design  of  the  Manhattan  Bridge  with  eye-bar  chains.  The  specifications 
for  both  bridges  having  been  written  at  the  same  time,  it  would  have  been 
surprising  had  the  specified  loads  not  been  the  same. 

The  following  is  the  paragraph  of  the  report  on  the  Manhattan  Bridge 
concerning  the  live  loads  for  truss  members: 

"The  maximum  congested  load  which  could  possibly  be  brought 
upon  this  bridge  would  consist  of  a  continuous  train  of  rapid  transit 
cars  on  each  of  the  four  elevated  tracks,  of  a  continuous  line  of  trolley 
cars  on  each  of  the  four  trolley  tracks;  of  a  crowd  of  heavy  tearns  on  the 
roadway  and  of  a  crowd  of  people  on  the  footway.  The  heaviest  rapid 
transit  train  w^hich  may  run  over  this  bridge  is  that  adopted  by  the 
Interborough  Company  for  the  subway;  such  a  train,  in  which  two- 
thirds  of  the  cars  are  motor  cars  (corresponding  to  the  practice  of  the 
Manhattan  Railway),  with  120  passengers  on  each  car,  is  estimated 
to  have  a  possible  weight  of  1,700  pounds  per  linear  foot  and  an 
extreme  axle  load  of  26,000  pounds.  The  estimated  maximum  weight 
of  a  continuous  line  of  trolley  cars  is  1,000  pounds  per  linear  foot.  The 
estimated  greatest  possible  congested  weight  per  linear  foot  on  this 
bridge  would  then  be  as  follows: 

4  rapid  transit  trains,  at  1,700  pounds    6,800  pounds 

4  lines  of  trolley  cars,  at  1,000  pounds    4,000  pounds 

35.5  feet  of  roadway,  at  100  pounds  per  square  foot    3,550  pounds 

22  feet  of  footway,  at  75  pounds  per  square  foot 1,650  pounds 

Total 16,000  pounds 

"  This  is  a  possible  load  which  could  never  occur  unless  special  pains 
were  taken  to  produce  it.  The  conditions  which  would  block  the  tracks 
with  continuous  lines  of  cars  in  one  direction  would  probably  prevent 
cars  entering  the  bridge  from  the  other  direction.  The  weight  on  the 
sidewa  ks  would  correspond  to  about  twelve  people  per  linear  foot, 
or  something  like  35,000  people  on  the  bridge.  The  congestion  of  the 
roadway  would  be  such  that  teams  could  not  move.  Under  these  con- 
ditions we  consider  that  one-half  of  this  amount. may  be  taken  as  a 


CHIEF   ENGINEER'S   REPORT  17 

maximum  working  load;  this  would  make  the  working  load  8,000  pounds 
per  linear  foot,  ivhich  is  three  times  that  provided  in  the  plans  of  the 
Brooklyn  Bridge  and  40  per  cent,  greater  than  that  taken  for  the  Williams- 
burg Bridge.^ 

"The  provision  of  2,000  pounds  per  linear  foot  for  wind  pressure 
proposed  by  the  Commissioner  is  ample. 

"In  calculating  the  effect  of  temperature,  provision  has  been  made 
for  an  extreme  variation  of  110  degrees  Fahrenheit,  which  we  consider 
sufficient. 

"  We  consider  that  the  bridge  should  be  so  proportioned  that  with 
the  congested  load  of  16,000  pounds  per  linear  foot,  covering  the  whole 
bridge,  combined  with  dead  load  and  wind  pressure,  no  stresses  would 
be  produced  anywhere  reaching  the  elastic  limit  of  the  material  or  im- 
pairing the  stability  of  the  anchorages.  In  other  words,  it  should  not 
be  possible  for  such  extraordinary  congested  load  to  do  any  permanent 
injury  to  the  bridge. 

"  We  consider  that  the  working  load  of  8,000  pounds  per  linear  foot 
should  be  used  in  designing  the  main  members  of  the  structure." 

[The  itahcs  are  ours. — F.  C.  K.] 

The  Commission  on  the  Manhattan  Bridge  did  not  specify  any  unit  stresses  The  Specified 
under  the  action  of  the  two  kinds  of  live  load  except  that  "such  extraordinary  ^^'^  stresses 
congested  load  should  not  do  any  permanent  injury  to  the  bridge."  The 
specifications  for  the  Blackwell's  Island  Bridge,  prescribed  for  tension  of  truss 
members  20,000  pounds  per  square  inch  for  "regular'"  or  working,  and  24,000 
pounds  per  square  inch  for  "congested"  or  extraordinary  live  load,  with  more 
than  the  usual  reduction  for  compression,  viz.,  20,000-90  l/r  and  24,000-100  l/r 
respectively.  [Messrs.  Boiler  &  Hodge  and  Professor  Burr,  in  their  reports, 
reduce  their  permissible  unit  stress  for  compression  according  to  formula 
20,000-50  l/r;  many  engineers  allow  for  compression  up  to  "I  over  r"  equal 
40  and  even  50,  nearly  or  exactly  the  same  unit  stress  as  for  tension.]  Pro- 
fessor Burr  calls  these  stresses  "safe  and  satisfactory  in  the  light  of  knowledge 
and  precedent  available  when  they  were  drawn"  (in  1903)  and,  since  the 
amount  of  the  "regular"  and  the  "congested"  live  load  was  also  specified 
and  actually  used  in  the  calculations,  it  would  only  remain  to  explain  how  this 
live  load  was  placed  to  obtain  the  greatest  stresses. 

The  specifications  stated  that  the  live  load  should  be  "placed  so  as  to  Distribution 
give  the  greatest  strain  in  each  part  of  the  structure."    This  is  the  usual   °conthiuous" 
wording  used  in  specifications  for  bridge  work.    For  ordinary  trusses,  the  live  or  "Discon- 
load  causing  the  greatest  strain  or  stress  in  each  truss  member  covers  one 
continuous,  i.  e.,  uninterrupted  stretch.    However,  in  arches,  most  types  of 
suspension  bridges,   like  the  Brooklyn  and  the  Manhattan  Bridges,  swing 
bridges  or  trusses  with  several  supports,  like  the  Blackwell's  Island  Bridge, 
the  live  load  for  the  greatest  theoretical  stress  in  some  members  would  have 

♦Brooklyn  Bridge  2,600  pounds,  Williamsburg  Bridge  5,700  pounds  for  the  cables  and  4,500  pounds 
for   the  stiffening  trusses,  all  figures  per  linear  foot  of  bridge. 


tinuous" 


18 


CHIEF    ENGINEER'S   REPORT 


Comp, 


to  cover  certain  disconlinuous,  i.  e.,  isolated  stretches,  different  for  different 
groups  of  members,  with  absolutely  no  live  load  between  these  stretches 
and  on  the  other  parts  of  the  bridge.  Any  live  load  outside  of  these  isolated 
stretches  would  reduce  the  greatest  stress  in  the  truss  members. 

To  illustrate  this,  three  influence  lines  are  shown  in  the  accompanying 
figure;  the  first  for  the  stress  in  a  diagonal  of  a  swing  bridge  on  three  sup- 
ports, the  second  for  the  stress  in  a  top  chord  member  near  the  center  of 
an  arch  with  two  hinges,  and  the  third  for  the  stress  in  a  top  chord  member 
of  the  island  span  of  the  Blackwell's  Island  Bridge.    A  live  load  in  the  stretch 

ab  or  cd  will  cause  tension  in  the 
diagonal  or  the  top-chord  members 
respectively,  and  a  live  load  in  the 
stretch  be  will  cause  compression  in 
these  members.  The  absolute  great- 
est tension,  therefore,  would  be 
caused  by  ab  and  cd  loaded  and  be 
unloaded. 

The  usual  method,  however,  is  to 
assume  a.  continuous,  i.e.,  uninterrupted 
stretch,  loaded  as  follows: 
If  area  B  is  smaller  than  area  C,  then 
load  from  a  to  d;  if  area  B  is  greater 
than  area  C,  then  load  from  a  to  b; 
in  other  words,  choose  that  uninter- 
rupted loaded  stretch  (ab  or  ad)  which- 
ever gives  the  greatest  stress.  This 
method  was  followed  in  the  design  of 
the  Blackwell's  Island  Bridge.  It  is 
the  usual  method,  even  in  single-track 
railroad  bridges,  where  one  train  may 
cover  cd  and  another  ab.  The  stresses 
in  the  arch  bridge  of  840  feet  span  at  the  Niagara  Falls  were  calculated  in  a 
similar  manner.  For  the  Washington  arch  bridge  in  New  York  City,  the 
stresses  were  determined  for  a  live  load  covering  the  whole  span  (510  feet) 
and  a  live  load  covering  the  stretch  from  abutment  to  the  center  of  span 
only,  and  the  greater  stresses  considered. 


Tens/or? 
Compression 


Compress/on 


Live  Load 
Stresses 


In  March,  1904,  the  contractor  was  furnished  with  the  "Loading  Key" 
(dated  January  26th,  1904),  prepared  by  the  Department  of  Bridges.  (See 
paper  by  Mr.  R.  C.  Strachan,  Assistant  Engineer  of  the  Department  of  Bridges, 
published  in  the  Proceedings  of  the  Engineers'  Club  of  Brooklyn,  1905; 
also  in  "  Engineering  News,"  February  16th,  1905,  in  which  the  analytical 
method  of  computation  is  described;  also  paper  by  Mr.  F.  H.  Cilley,  in  the 
Transactions  of  the  American  Society  of  Civil  Engineers,  1904,  describing 
the  graphical  method  used  for  checking  the  live  load  stresses.)  This  "Load- 
ing Key"  gives  the  influence  on  the  stresses  in  the  different  truss  members 


CHIEF    ENGINEER'S   REPORT  19 

of  a  live  load  of  1,000  pounds  per  foot  per  truss,  covering  different  adjoining 
stretches  of  the  bridge. 

The  following  is  quoted  from  Mr.  Strachan's  paper: 

"The  calculations  were  made  by  the  writer  for  the  Department 
of  Bridges  of  the  City  of  New  York  during  the  years  1902  and  1903, 
and  have  more  recently  been  revised  to  conform  to  altered  data. 
"The  subsequent  addition  of  two  elevated  railroad  tracks,  giving 
a  total  "congested"  live  load  of  8,000  pounds,  or  "working  load" 
of  4,000  pounds  per  foot  per  truss,  made  necessary  a  revision  of  both 
dead  and  live  load  stresses;  and  the  results  herein  given  are  based  upon 
the  latter  load. 

"The  connection  of  the  cantilever  arms  at  mid-channel  causes 
a  load  on  any  subdivision  of  the  bridge  to  produce  stresses  in  all  the 
others;  the  effect,  however,  upon  the  east  truss  of  a  load  on  the  west 
truss,  or  vice  versa,  is  generally  small,  as  will  appear  when  the  results 
are  examined.  Moreover,  the  simultaneous  and  complete  stoppage 
of  all  the  many  lines  of  traffic  in  such  a  way  as  to  give  full  live  load 
on  two  widely  separated  subdivisions  (divisions  2  and  7,  for  exam- 
ple), and  nowhere  else,  is  practically  out  of  the  question,  as  is  the 
assumption  of  live  load  covering  any  short  isolated  portion  of  the 
bridge. 

"Stresses  for  seven  positions  of  live  load  were  therefore  calcu- 
lated, the  load  in  each  case  covering  one  of  the  seven  subdivisions; 
and  the  maxima,  both  tensile  and  compressive,  obtained  by  com- 
bining these  for  a  continuous  load  extending  over  one  or  more  sub- 
divisions, are  the  governing  stresses  for  the  main  members  of  the 
truss. 

"  In  the  absence  of  any  similar  truss  with  which  to  compare,  weights 
were  obtained  by  successive  approximation,  after  the  study  of  a 
sufficient  number  of  members  of  each  class  had  made  possible  a  reli- 
able estimate  of  the  percentages  for  details.  Those  stresses,  due  to 
either  dead  or  live  load,  which  are  unaffected  by  the  action  of  the 
rockers,  were  obtained  graphically  and  checked  by  analytical  methods, 
more  especially  to  detect  errors  in  scaling,  since  the  proper  closing 
of  the  graphic  diagrams  constitute  a  check  upon  the  graphical  work 
itself.  The  results  of  the  action  of  the  rockers  were  ascertained  ana- 
lytically. 

"  We  thus  have  for  each  position  of  load  what  may  be  called  the 
partial  stresses.  The  greatest  tension  and  compression  from  a  load 
extending  over  one,  two  or  more  continuous  subdivisions,  obtained 
by  the  algebraic  addition  of  the  partial  stresses  for  each  member, 
are  then  tabulated." 

The  following  diagram  explains  the  continuous  and  discontinuous  load- 
ing  for  a  few  members. 


CHIEF    ENGINEER'S   REPORT  21 

It  may  be  of  interest  to  mention  that,  in  the  discussion  which  followed 
the  reading  of  this  paper,  and  in  which  Messrs.  Nichols  and  R.  S.  Buck  par- 
ticipated, and  the  correspondence  sent  in  by  Messrs.  Duryea,  Mayer,  Hodge 
and  Moisseiff,  no  mention  was  made  of  the  difference  of  the  effect  of  a  live 
load  iii' continuous  stretches  as  compared  with  that  of  a  live  load  in  discon- 
tinuous stretches. 

Mr.  Henry  W.  Hodge  writes  in  his  discussion: 

"I  have  taken  great  pleasure  in  reading  Mr.  Strachan's  paper 
on  the  "Computation  of  Stresses  in  the  Blackwell's  Island  Bridge," 
and  his  method  of  arriving  at  these  stresses  in  the  rocker  members 
connecting  the  cantilever  arms  is  as  simple  and  accurate  as  any  method 
can  be  which  is  based  on  deflection  of  framed  structures. 

"As  far  as  the  computation  of  stresses  in  this  structure  is  con- 
cerned, there  is  little  discussion  possible;  but,  as  to  the  wider  ques- 
tion of  the  advisability  of  constructing  a  cantilever  structure  on 
this  semi-continuous  design,  engineers'  opinions  vary  considerably." 

Mr.  Joseph  Mayer  writes: 

"The  method  given,  which  is  based  on  that  used  in  the  accurate 
calculation  of  drawbridge  stresses  is,  I  believe,  the  only  accurate 
method  available.  I  believe  the  stresses  and  sections  required  in  the 
absence  of  rockers  will  give  good  preliminary  data  for  calculating 
the  deflections  produced  by  given  forces  at  the  rockers.  Your  paper 
is  so  faultless  that  it  does  not  lend  itself  to  much  discussion." 

Mr.  Edwin  Duryea,  Jr.,  referring  in  his  letter  to  the  assumption  as  to 
"probable"  maximum  live  load,  mentions  that  a  "more  rational  method" 
of  estimating  probable  maximum  load  had  been  used  by  Mr.  Joseph  Mayer 
in  designing  a  railway  bridge  across  the  North  River.  In  looking  this  up 
in  the  Transactions  of  the  American  Society  of  Civil  Engineers,  Vol.  48, 
1902,  p.  375,  we  find: 

"For  the  purpose  of  comparing  the  merits  of  various  types  for 
a  bridge  across  the  Hudson,  the  writer,  therefore,  will  use  a  bridge 
having  twelve  tracks,  of  which  two  are  for  freight  trains,  two  for 
long-distance  passenger  trains,  six  for  trains  of  the  elevated  and 
underground  roads  of  New  York  City  and  two  for  surface  cars. 

"Such  a  bridge  has  sufficient  capacity  for  the  business  it  can  secure 
in  the  near  future.  The  distant  future  may  bring  other  competing 
bridges  or  tunnels,  therefore  it  need  not  be  considered. 

"The  moving  load  would  consist  of  two  freight  trains,  each  1,000 
feet  long,  weighing  3,000  pounds  per  linear  foot;  two  long-distance 
passenger  trains,  each  1,000  feet  long  and  weighing  1,500  pounds 
per  linear  foot;  six  r£[pid  transit  electric  trains,  each  500  feet  long 
and  weighing  1,200  pounds  per  linear  foot. 

"The  surface  cars,  on  the  two  tracks  provided  for  them,  should 


22  CHIEF   ENGINEER'S   REPORT 

run  at  a  speed  of  at  least  15  miles  per  hour.  They  would,  therefore, 
be  twice  as  far  apart  as  on  the  street  approach;  a  distance  of  100 
feet  from  center  to  center  of  cars  would  be  closer  than  practicable. 
This  distance,  with  cars  weighing  40,000  pounds,  gives  a  load  of  400 
pounds  per  linear  foot  of  track. 

"For  the  calculation  of  the  cables,  anchorages  and  the  towers 
above  the  floor  level,  this  load  is  equivalent  to  8,421  pounds  per  linear 
foot  of  bridge,  covering  the  whole  length  of  the  main  span.  The  writer, 
therefore,  will  assume  a  moving  load  of  8,500  pounds  per  linear  foot 
of  bridge  for  these  calculations. 

"For  the  calculation  of  the  stiffening  trusses,  the  loads  on  the 
surface-car  tracks  may  be  neglected,  as  they  are  nearly  uniformly  dis- 
tributed over  the  length  of  the  bridge.  The  stresses  produced  by  the 
trains  1,000  feet  long  and  those  produced  by  the  trains  500  feet  long 
would  have  to  be  calculated  separately  and  then  added,  if  the  exact 
stresses  corresponding  to  these  loads  are  wanted.  No  equivalent 
load  of  one  length  will  give  the  same  stresses  in  every  member  of  the 
stiffening  trusses  as  the  two  loads  of  different  lengths." 

[The  italics  are  ours. — F.  C.  K.] 

The  working  live  load  of  8,500  pounds  per  linear  foot,  representing  ten 
railway  tracks,  two  of  which  for  heavy  freight  and  two  trolley  lines,  is  therefore 
about  the  same  as  the  8,000  pounds  working  or  "regular"  specified  in  1903 
for  the  Blackwell's  Island  Bridge,  based  on  a  weight  of  rapid  transit  trains 
of  1,700  pounds  per  linear  foot,  in  place  of  Mr.  Mayer's  1,200,  and  of  trolley 
cars  weighing  43,000  pounds  each,  in  place  of  Mr.  Mayer's  40,000.  (The 
reports  of  Messrs.  Boiler  &  Hodge  and  Professor  Burr  give  1,810  pounds 
per  linear  foot  and  62,000  pounds  respectively,  which  is  an  increase  of  50 
per  cent,  in  seven  years.)  It  is  also  interesting  to  note  that  Mr.  Mayer,  for 
a  bridge  of  2,800  feet  span,  assumes  on  each  of  the  ten  railway  tracks  only 
one  train  of  1,000  feet  or  500  feet  length  respectively,  and  disregards  the 
possibility  of  partial  "bunching"  of  trolley  cars  on  the  two  tracks  provided 
for  them. 

The  method  of  probabilities  mentioned  by  Mr.  Duryea  as  having  been 
used  by  Mr.  Mayer  to  determine  the  probable  frequency  of  the  concurrence 
of  trains  with  the  maximum  load  on  two,  three,  four,  five,  six,  etc.,  tracks, 
assuming  the  trains  running  on  schedule  time  with  certain  constant  veloci- 
ties, could  not  be  employed  here,  as  even  the  most  elaborate  assumptions 
as  regards  speed  and  time  of  the  rapid  transit  trains  on  the  four  tracks  will 
be  often  overthrown  by  delays,  accidents,  etc.,  and  the  traffic  on  the  trolley 
tracks,  roadway  and  promenades  is  beyond  the  reach  of  a  mathematical 
expression  of  its  probability. 

The  following  is  quoted  from  "Engineering  News"  concerning  Mr.  0. 
.  F.   Nichols'   (at  that  time  Chief  Engineer  of  the  Department  of  Bridges) 
discussion  of  Mr.  Strachan's  paper: 


CHIEF   ENGINEER'S   REPORT  23 

"Speaking  then  in  some  detail  of  the  width  and  traffic  capacity 
of  the  structure,  Mr.  Nichols  expressed  the  opinion  that  the  roadway 
as  described  is  far  too  narrow  to  accommodate  the  street  traffic  which 
will  come  upon  it.  He  expects  that  the  roadway  will  be  overcrowded 
as  soon  as  the  bridge  is  opened.  The  provision  of  two  lines  of  ele- 
vated railway  as  originally  made  would  also  have  proved  a  serious 
error,  but  the  four  tracks  now  provided  will  suffice  for  some  years. 
One  or  two  details  of  the  structural  design  were  also  discussed  by 
the  speaker."    ("Engineering  News,"  1905,  Vol.  53,  p.  178.) 

In  this  connection,  it  may  also  be  of  interest  to  quote  from  Mr.  0.  S. 
Morison's  paper  (see  Transactions  of  the  American  Society  of  Civil  Engi- 
neers, Vol.  36,  1896)  regarding  the  assumed  live  load  for  a  suspension  bridge 
of  3,100  feet  span  over  the  Hudson  River  in  the  City  of  New  York,  even  if 
the  paper  was  written  twelve  years  ago: 

"The  bridge  has  been  designed  to  carry  a  total  load  of  25  tons, 
or  50,000  pounds,  per  lineal  foot.  The  design  has  then  been  devel- 
oped and  the  dead  weight  calculated,  and  the  result  is  a  balance 
for  the  live  load  of  11,000  pounds  per  foot  over  the  entire  structure. 
As  the  width  of  the  structure  is  92  feet  between  the  stiffening  trusses, 
this  corresponds  to  about  120  pounds  per  square  foot  of  floor.  If 
this  space  were  to  be  occupied  by  eight  railroad  tracks,  it  would 
amount  to  1,375  founds  per  lineal  foot  per  track,  which  exceeds  the 
weight  of  any  passenger  train.  It  would  amount  in  the  aggregate  on 
a  length  of  3,100  feet  to  34,110,000  pounds — equivalent  to  eight 
freight  trains  1,400  feet  long,  each  weighing  3,000  pounds  per  lineal 
foot.  It  is  probable  that  the  requirements  of  any  location  where  a 
bridge  of  this  magnitude  would  be  considered  would  be  satisfied  by 
four  railroad  tracks  adapted  to  a  heavy  class  of  traffic  and  four  rapid 
transit  tracks  to  be  operated  by  electric  cars  or  short  trains  of  a  char- 
acter which  would  require  only  a  floor  stiffener  to  secure  the  necessary 
rigidity.  Therefore,  in  proportioning  the  stiffening  truss,  the  varia- 
ble load  has  been  taken  on  the  basis  of  12,000  pounds  per  lineal  foot, 
corresponding  to  a  load  of  3,000  pounds  per  foot  on  each  of  the  rail- 
road tracks,  with  no  provision  for  unequal  weight  on  the  rapid  transit 
tracks,  or  to  1,500  pounds  per  Uneal  foot  on  all  eight  of  the  tracks. 
These  provisions  correspond  to  four  maximum  freight  trains  or  eight 
maximum  passenger  trains." 

[The  italics  are  ours. — F.  C.  K.] 

The  original  estimate  of  the  contract  of  the  Blackwell's  Island  Bridge, 
with  two  elevated  railroad  tracks,  made  in  1903,  was  about  86,000,000 
pounds;  the  addition  of  two  elevated  railroad  tracks  to  the  capacity  of  the 
bridge,  with  changes  and  additions  made  by  the  Department  of  Bridges 
in  1904,  as  well  as  more  specific  knowledge  in  regard  to  the  probable  weight 
of  details,  indicated  that,   with  the  Uve  load  in  continuous  stretches,  the 


24.  CHIEF   ENGINEER'S   REPORT 

total  steel  weight  would  run  up  to  about  100,000,000  pounds,  with  a  great- 
est cross-section  of  the  bottom  chord  of  the  trusses  equal  to  about  1,100 
square  inches,  while,  for  the  live  load  in  discontinuous  stretches,  the  total 
steel  weight  would  be  about  10  per  cent,  and  the  chords  about  25  per  cent, 
heavier,  with  all  the  accompanying  difficulties  of  excessive  thickness  of 
material,  inadequate  space  for  the  lacing  of  bottom  chord  and  for  the  pack- 
ing on  the  pins,  etc. 

The  question  of  continuity  or  discontinuity  of  the  live  load,  therefore, 
involved  a  question  of  an  increase  of  the  steel  weight  of  the  structure  of  about 
10,000,000  pounds,  at  a  corresponding  additional  cost  of  approximately 
$700,000. 

The  adoption  of  the  continuous  live  load  by  the  Department  of  Bridges 
was  based  on  the  unlikelihood  that  the  fourteen  lines  of  live  load,  consisting 
of  four  kinds  of  traffic,  i.  e.,  rapid  transit  trains,  trolley  cars,  wagons  and 
pedestrians,  on  two  independent  floors,  would  be  distributed  simultaneously 
in  the  following  manner,  viz.: 

First — Each  kind  of  traffic  up  to  its  assumed  "regular"  or  "congested" 
maximum  per  linear  foot ; 

Second — Each  separate  kind  of  traffic  in  two  or  three  isolated  stretches 
only,  these  stretches  being  in  certain  exact  distances  from  each  other  and 
from  the  ends  of  the  bridge ; 

Third — These  stretches  and  distances  to  be  alike,  i.  e.,  in  the  same  loca- 
tion for  all  the  fourteen  lines  of  traffic  on  the  two  floors ; 

Fourth — Absolutely  no  other  live  load  on  the  bridge. 

Such  conditions  of  loading  could  scarcely  be  produced,  even  as  an  experi- 
ment, since  trains,  cars,  wagons  and  horses  each  cover  certain  definite  spaces. 

The  Department  of  Bridges,  therefore,  adopted  in  1904  the  following 
conditions  for  designing  the  truss  members,  Nos.  1  and  2  providing  for  con- 
tinuous live  loads,  while  Nos.  3  and  4  take  account  of  extraordinary  con- 
ditions that  might  arise  from  discontinuous  loading,  viz.: 

(1)  The  "regular"  five  load  of  8,000  pounds  per  finear  foot  of 
bridge  in  one  continuous  stretch  for  each  member,  over  such  a  portion 
of  the  bridge  as  to  cause  maximum  stress,  and  using  20,000  pounds 
per  square  inch  in  tension  (reduced  for  compression)  for  structural 
and  30,000  pounds  per  square  inch  in  tension  for  nickel-steel  as  per- 
missible without  wind  acting,  and  increased  20  per  cent,  with  wind 
acting. 

(2)  The  "congested"  five  load  of  16,000  pounds  per  linear  foot 
of  bridge  in  one  continuous  stretch  for  each  member,  over  such  a  por- 
tion of  the  bridge  as  to  cause  maximum  stress  without  wind,  using 
24,000  pounds  per  square  inch  in  tension  (reduced  for  compression) 
for  structural  and  39,000  pounds  per  square  inch  in  tension  for  nickel- 
steel   as  permissible. 

Of  these  two  conditions,  the  one  causing  the  greater  cross  section  in 
each  truss  member  should  be  taken. 


CHIEF    ENGINEER'S   REPORT 


25 


(3)  If  the  "congested"  live  load  of  1G,000  pounds  per  linear  foot 
of  bridge  in  discontinuous  stretches  (without  wind  acting)  causes 
an  absolute  maximum  compression  greater  than  the  dead  load  ten- 
sion in  a  member,  such  member  to  be  made  of  a  section  able  to  with- 
stand the  resulting  compression. 

(4)  Furthermore,  in  order  to  be  sure  that  no  overstressed  con- 
dition could  result  from  the  "regular"  live  load  of  8,000  pounds 
per  linear  foot  of  bridge,  in  discontinuous  stretches  causing  absolute 
maximum  stress  without  wind,  the  unit  stresses  are  to  be  analyzed 
for  this  condition. 

For  this  last  condition,  the  greatest  unit  stress  in  tension,  with  two  excep- 
tions, would  not  be  greater  than  20,700  pounds,  and  in  compression  18,800 
pounds  for  l/r=27  for  structural  steel  and  34,100  pounds  in  tension  for 
nickel-steel,  the  exceptions  being  the  verticals  U59-L59  and  U73-L73,  which 
are  stressed  in  tension  to  22,900  (see  plate  No.  5). 

The  combination  of  live  load  with  the  greatest  wind  pressure  was  con- 
sidered for  condition  (1)  only,  and  the  unit  stresses  increased  by  the  usual 
amount  of  20  per  cent.,  since  it  is  hardly  possible  that  the  bridge  will  be 
crowded  on  the  roadway  and  the  promenades  in  stormy  weather.  A  con- 
currence of  a  great  wind  pressure  with  conditions  (2),  (3)  or  (4)  is  practi- 
cally impossible. 

The  wind  pressure  was  assumed  at  2,000  pounds  per  linear  foot  of  bridge, 
representing  about  35  to  40  pounds  per  square  foot,  which  would  be  caused 
by  a  velocity  of  wind  of  100  miles  per  hour;  and  it  is  clear  that  in  the  same 
degree  as  the  wind  pressure  increases,  the  live  load  on  the  roadway,  prome- 
nades, etc.,  naturally  decreases,  which  is  distinctive  of  highway  bridges 
as  compared  with  railroad  bridges,  since,  in  the  latter,  the  maximum  live 
loads  will  pass  irrespective  of  the  velocity  of  the  wind. 

Attached  plates  Nos.  1  and  2  show  the  position  of  the  live  load  in  con- 
tinuous stretches  as  assumed  by  the  Department  of  Bridges  for  the  design,  and 
plates  Nos.  3  and  4  the  live  load  in  discontinuous  stretches  as  assumed  by 
the  experts  for  the  report.  Heavy  full  lines  indicate  loading  for  greatest  ten- 
sion; dotted  Unes,  loading  for  greatest  compression  in  each  member. 

To  illustrate:  For  the  top  and  bottom  chord,  in  fact  for  most  truss  mem- 
bers of  the  Island  span,  where,  according  to  the  reports  of  the  experts,  the 
highest  stiiesses  occur,  the  continuous  "congested"  loading  covers  the  distance 
between  piers  Nos.  1  and  4;  i.  e.,  both  river  spans  and  the  Island  span,  a 
total  length  of  2,796  feet,  while  for  the  discontinuous  loading  the  "congested" 
load  would  cover  the  distance  of  1,182  feet  between  piers  Nos.  1  and  2,  no 
live  load  whatever  for  a  distance  of  630  feet  between  piers  Nos.  2  and  3, 
and  again,  the  "congested"  live  load  for  a  distance  of  984  feet  between 
piers  Nos.  3  and  4;  whether  loading  is  continuous  or  discontinuous,  both 
anchor  spans  have  to  be  absolutely  free  of  any  live  load.  The  possibility 
that  the  "congested"  Uve  load  of  16,000  pounds  per  linear  foot  of  bridge, 
representing  fourteen  lines  of  traffic  of  four  different   kinds  on  two  inde- 


Distribution 
of  Live  Load 
for  the  Great- 
est Stresses  in 
the  Island 
Span  (Com- 
pare Table 
No.  7) 


26  CHIEF   ENGINEER'S   REPORT 

pendent  floors  of  uniform  construction  from  end  to  end  of  the  bridge,  will 
cover  1,182  feet,  is  indeed  very  small;  that  another  bunching  of  traffic  on  all 
fourteen  lines  on  the  two  floors  precisely  in  the  same  location  for  a  length 
of  984  feet  will  occur,  is  still  smaller;  but  what  can  be  the  possibility  that 
these  two  loaded  stretches  will  be  630  feet  apart  with  no  live  load  on  the 
two  floors  between  them,  a  distribution  which  even  for  the  working  live  load 
of  a  single-track  railroad  bridge  would  be  exceptional? 

It  would  not  be  surprising  if  some  engineers,  differing  honestly  in  their 
judgment  from  that  of  the  designers,  would  reverse  the  question  and  claim 
that  the  "congested"  five  load  of  16,000  pounds  per  Unear  foot  of  bridge, 
even  in  a  continuous  stretch  of  2,796  feet,  representing  a  distribution  of 
cars,  wagons,  pedestrians,  as  shown  on  plate  No.  7,  from  piers  Nos.  1  to  4, 
and  no  other  live  load  on  the  two  floors  from  these  piers  to  the  abutments, 
is  a  practical  impossibility;  that  therefore  the  Island  span  has  been  waste- 
fully  designed  by  the  Department  of  Bridges  and  that  the  engineers  of  the 
contractor  must  have  known  this  and  should  have  protested  even  if  the  bridge 
was  paid  for  by  the  pound. 

In  order  to  represent  on  plate  No.  7  the  "congested"  load  of  16,000  pounds 
per  linear  foot  of  bridge,  it  was  necessary  to  place: 

First — On  each  of  the  four  trolley  tracks  an  uninterrupted  line  of  trolley 
cars  of  1,000  pounds  per  linear  foot  of  track  ; 

Second — On  each  of  the  four  rapid  transit  tracks,  an  uninterrupted 
line  of  trains  of  1,700  pounds  per  linear  foot  of  track ; 

Third — On  the  two  promenades,  a  crowd  of  people  weighing  75  pounds 
per  square  foot,  and 

Fourth — On  the  roadway  (35^  feet  wide)  four  uninterrupted  lines  of 
the  heaviest  electric  trucks  known  to  us,  of  7  feet  by  18  feet  net  area  and 
weighing  loaded  18,000  pounds,  all  assumed  carrying  their  maximum  load 
and  placed  two  feet  apart  in  the  clear.  It  is  questionable  whether  two  feet 
clear  space  is  sufficient,  but  this  would  be  the  only  possibility  to  reach  a  live 
load  of  100  pounds  per  square  foot  on  the  roadway.  The  heaviest  electric 
trucks  used  in  New  York  City  (20,000  pounds,  with  8  feet  by  20  feet  net 
area)  and  the  heaviest  coal  wagons  (19,000  pounds,  drawn  by  three  horses 
of  1,800  pounds  each,  and  9  feet  3  inches  by  25  feet  total  net  area)  would 
give  considerably  smaller  load  per  linear  foot  of  bridge,  being  not  only  lighter 
per  square  foot,  but  also  wider,  so  that  the  roadway  between  the  trolley 
tracks  could  accommodate  only  three  lines.  Considering  further  that  in  the 
great  lengths  of  live  load  necessary  to  cause  maximum  stresses  in  the  main 
members  of  the  trusses,  there  will  be  different  kinds  of  vehicles  weighing 
considerably  less  (the  heaviest  touring  car  weighs  4,800  pounds,  with  a  net 
area  of  5  feet  6  inches  by  13  feet,  which  gives  43  pounds  per  square  foot, 
assuming  two  feet  clear  space),  especially  since  the  bridge  is,  for  its  greater 
part,  on  a  grade  of  3^  per  cent.,  and  that  there  will  be  many  large  floor  spaces- 
empty  on  account  of  irregular  dimensions  of  the  vehicles,  it  would  seem 
that  a  live  load  of  even  50  pounds  per  square  foot  of  roadway  could  hardly 
be  reached. 


CHIEF   ENGINEER'S   REPORT  27 

The  "regular"  live  load  of  the  specifications  (8,000  pounds  per  linear 
foot  of  bridge,  represented  by  37.5  pounds  per  square  foot  on  the  promenades, 
50  pounds  per  square  foot  on  the  roadway,  4X500=2,000  pounds  per  linear 
foot  of  bridge  for  the  trolley  tracks,  and  4X850=3,400  pounds  per  linear 
foot  of  bridge  for  the  rapid  transit  tracks)  takes  into  account  only  one-half 
of  the  density  of  the  "congested"  live  load  (shown  for  most  members  of 
the  Island  span  on  plate  No.  7),  simply  assuming  a  greater  space  between 
the  small  units  represented  by  pedestrians,  wagons,  trolley  cars  and  the 
trains.  This  requires  no  further  explanation  except  for  the  rapid  transit 
trains. 

In  the  "congested"  live  load  the  average  weight  of  a  train  is  assumed  influence  of 
at  1,700  pounds  per  linear  foot;  since  of  this  amount  about  three-fourths  xr^ains^iT"^^' 
is  for  the  cars  and  one-fourth  for  the  passengers,  it  is  clear  that  a  smaller  stresses 
number  of  passengers  reduces  the  weight  of  the  trains  only  slightly,  so  that 
their  share  of  6,800  pounds  in  the  "congested"  live  load  of  16,000  pounds 
per  linear  foot  of  bridge  cannot  be  reduced  materially  for  their  share  in  the 
"regular"  live  load  of  8,000  pounds  per  Unear  foot  of  bridge.    However, 
there  are  other  more  important  considerations  which  make  it  permissible 
to  assume  that  the  stresses  due  to  the  trains  in  the  "congested"  live  load  of 
16,000  pounds  per  linear  foot  of  bridge  can  be  reduced  one-half  to  repre- 
sent those  due  to  the  trains  in  the  "regular"  live  load,  viz.: 

First — The  trains  are  limited  to  a  certain  length  (express  trains 
about  400  feet  long,  with  eight  cars,  and  local  trains  about  250  feet 
long,  with  five  cars),  so  that  they  produce  in  most  members  only  part 
of  the  stress  which  would  be  produced  by  the  lengths  required  for 
the  maximum  stress,  these  lengths  being,  as  a  rule,  considerably 
greater. 

Second — The  trains  following  on  the  same  track  have  to  main- 
tain, on  account  of  safety  of  operation,  a  clear  interval  between  them 
of  not  less  than  2,400  feet  (for  the  stresses  in  the  Island  span,  for 
instance,  no  two  trains  can  therefore  be  at  the  same  time  on  the  same 
track  between  piers  Nos.  1  and  4). 

Third — The  probability  that  all  the  trains  on  the  four  tracks 
will  pass  simultaneously  through  the  danger  zones  where  they  con- 
tribute their  maximum  share  to  the  stress  in  a  truss  member  is  very 
small,  as  a  rule  they  will  be  more  uniformly  distributed  over  the 
bridge. 

Should  it,  however,  happen  that,  through  some  accident,  two  trains  on 
the  same  track  would  follow  each  other  in  a  shorter  clear  distance  than  2,400 
feet  (for  the  greatest  stress  in  the  Island  span  about  1,300  feet  clear),  the 
probability  that  each  of  the  other  three  tracks  will  be  loaded  by  two  trains 
following  each  other  in  the  same  clear  distance,  that  these  two  groups,  each 
of  four  trains,  will  pass  at  the  same  time  through  their  respective  danger 
zones  (for  the  Island  span,  four  trains  in  the  middle  of  the  one  river  span. 


28  CHIEF   ENGINEER'S   REPORT 

and  four  trains  in  the  middle  of  the  other  river  span),  and  at  the  same  time 
that  the  other  three  kinds  of  traffic  of  "regular"  density  weighing  8,000 
minus  3,400=4,600  pounds  per  linear  foot  of  bridge  will  cover  even  the 
continuous  lengths  necessary  to  produce  the  maximum  stresses,  is  so  remote 
that  this  combination  of  the  train  loads  with  the  other  loading  should  be 
considered  an  extreme  condition,  and  unit  stresses  allowed  somewhere  between 
the  permissible  unit  stress  for  the  "regular"  and  for  the  "congested"  live 
load.  As  a  matter  of  fact,  this  condition  would  cause  a  unit  stress  in  tension 
in  the  vertical  U59-L59  of  approximately  21,200  pounds  and  in  the  top 
chord  U65-U69  (nickel-steel)  of  approximately  33,500  pounds. 

The  examples  in  the  following  table  show  the  influence  on  the  stresses 
due  to  a  load  of  4X1,700=6,800  pounds  per  Hnear  foot  of  bridge  of  vari- 
ous length  and  location.  (6,800  pounds  is  the  assumed  train  load  on  the 
four  tracks  per  foot  of  bridge.) 


8 

5 

^ 

1 

i^ 

^ 

5 

•0 

1 

1 

1 

1 

1 

1 

1- 

1 

1 
^ 

1 

^ 

i 

^ 

^ 

i?5 

5i 

1 

<5i 

1 

1 

? 

1 

1- 

^ 
? 

1 

1 

K§ 

1 

^ 

1 

?J 

5! 

§ 

12 

SJ 
•n 

1 

1 

1- 

1 

15 

1 

5 
;? 
^ 

1 

S 
^ 

'VJ 


wi 


LJ 


L-^ 


»<) 


I 


I 


s 


1 

5? 


§l| 


^^^ 

^ 


I 


I 


I 


^^ 


30  CHIEF   ENGINEER'S   REPORT 

The  stresses  from  the  410-ft.  trains  are  only  approximate;  the  influence 
lines  were  not  given  us  by  the  Department  of  Bridges,  the  "Loading  Key" 
indicating  only  the  totals  for  certain  stretches  of  the  live  load. 

stresses  in         These  are  the  two   members  in  the  truss  which,   for  the  "congested" 
TT..«  7^i^^^^^l  discontinuous   live   load,    make   the   most   unfavorable   showing,    since  they 

U59-L59    and  •  i        j 

U73-L73  get  the  greatest  tension  for  the  same  condition  of  loading  as  the  top  chord 
in  the  Island  span.  Their  greatest  unit  stress  in  tension  for  dead  and  maxi- 
mum live  load  is  (see  table  No.  5): 

for   the   "regular"    continuous   live   load,    18,600   pounds; 
for  the  "regular"  discontinuous  Uve  load,  22,900  pounds; 
for  the  "congested"   continuous  Uve  load,   25,600   pounds.; 
and  would  be  for  the  "congested"  discontinuous  live  load,  34,200  pounds. 

If  we  assume  a  position  of  Uve  load  which  is  more  probable  than  the 
"regular"  discontinuous  loading,  viz.,  4,000  pounds  per  linear  foot  of  bridge 
covering  the  bridge  from  end  to  end,  and  additional  4,000  pounds  per  Unear 
foot  in  discontinuous  stretches,  we  get  a  total  unit  stress  of  20,300  pounds. 

If  we  assume  a  load  of  8,000  pounds  per  linear  foot,  covering  the  bridge 
from  end  to  end,  and  additional  8,000  pounds  in  discontinuous  stretches, 
merely  to  show  the  effect  of  a  milder  form  of  a  discontinuous  "congested" 
load,  we  get  29,000  pounds  per  square  inch. 

To  judge  these  stresses,  it  should  be  considered  that  verticals  have  no 
bending  stresses  from  own  weight  and  that  the  secondary  stresses  of  these 
verticals,  due  to  the  vertical  deformation  of  the  truss,  are  practically  zero, 
since  they  are  pin  connected  not  only  at  top  and  bottom,  but  also  at  the 
middle,   where  the   horizontal  strut   connects. 

Even  in  designing  these  verticals  for  condition  (2)  ("congested"  con- 
tinuous) as  this  gave  the  greater  stress,  their  packing  was  so  difficult  that 
their  upper  half  had  to  be  made  flared  at  the  top  in  order  to  minimize  the 
great  bending  moment  of  the  pin  caused  by  the  kink  in  the  outUne  of  the 
top  chord,  which  necessitated  a  pin  joint  in  their  center;  and  their  bottom, 
consisting  of  three  ribs,  each  five  inches  thick,  had  to  be  made  of  nickel-steel 
to  provide  for  the  necessary  area  through  and  back  of  the  pin  hole,  in  spite 
of  the  fact  that  the  bottom  chord  in  the  Island  span  is  ten  inches  wider  than 
in  the  rest  of  the  bridge. 

Difference  in         The  effect  of  the  discontinuous  live  load  on  the  stresses  compared  with 

Effect  of  Dis-  ^)^^^  Qf  ^^Q  continuous  live  load  is  different  according  to  the  location  of  the 

and  Continu-  truss  member.    In  the  Island  span  it  is  the  greatest,  in  other  members  the 

ous  Live  Load  difference  is  zero  or  approximately  zero,  as  for  instance  in  nearly  all  the 

Uniform  members  of  the  cantilever  arms,  in  the  bottom  chord  L58-L59  of  the  Island 

span,  etc.    This,  and  the  fact  that  in  every  bridge,  especially  of  long  span, 

there  are  certain  members  which  cannot  be  reduced  in  their  section  in  the 

same  proportion   as  the  stresses  decrease,   account  for  the  non-uniformity 

of  the  unit  stresses,   and,   therefore,   apparent  waste  of  material,   deduced 


CHIEF    ENGINEER'S   REPORT  31 

by  a  certain  Engineering  paper  from  the  reports  of  the  experts,  who  considered 
a  discontinuous  "congested"  live  load. 

As    mentioned    before,  the    condition     (3),  page  25,  that  is,  the  "con-  Provision 

gested"  discontinuous  live  load,  has  been  considered  for  reversal  of  tensile  *or  Reversal 
.  .  of  Stress  in 

stresses.    If  m  a  riveted  member  or  an  eye-bar  in  tension  the  elastic  limit  Tension 

or  yield  point  should  be  slightly  exceeded,  nothing  more  serious  than  an  Members 
imperceptible  permanent  set  would  occur  which  would  even  decrease  (not 
disappear)  if  a  certain  time  would  elapse  before  a  repetition  of  this  stress, 
since  the  yield  point  rises  with  an  excessive  stress.  [It  may  be  of  interest 
to  mention  in  this  connection  that  a  well-known  mechanical  engineer  pro- 
poses to  stress  above  the  yield  point  every  eye-bar  in  full  size  before  using 
it  in  the  structure  in  order  to  raise  its  yield  point.]  In  making  a  full-size  eye- 
bar  test,  the  yield  point  is  reached  within  the  first  minute,  while  it  may  take 
half  an  hour  of  increasing  pulling  to  reach  the  ultimate  strength  and  break 
the  bar.  If,  however,  an  eye-bar  were  only  slightly  compressed,  the  bridge 
might  fall.  For  this  reason,  some  top  chords  in  the  middle  of  the  river  spans 
and  at  the  ends  of  the  anchor  arms  and  some  diagonals  were  made  riveted 
members;  and  since  they  had  to  have  a  certain  "I  over  r"  to  be  stiff,  their 
unit  stress  is  naturally  low  and  their  section,  therefore,  seemingly  excessive. 
Taking  a  practically  impossible  load,  like  the  "congested"  discontinuous 
live  load  for  reversal  of  tensile  stresses,  but  not  for  determining  the  cross 
section  of  truss  members,  is  in  line  with  modern  specifications  for  railroad 
bridges,  one  of  the  foremost  railroads  in  the  country  using  for  a  working 
load  of  Cooper's  E50  a  unit  stress  in  tension  of  16,000  pounds  (consider- 
ing impact),  but  also  providing  for  an  extraordinary  live  load  of  Cooper's 
ElOO,  allowing  in  this  case  twice  the  unit  stress  for  the  working  load,  there- 
fore, for  tension,  32,000  pounds  per  square  inch  (elastic  limit),  in  order  to 
reach  those  truss  members  whose  stresses  could  be  reversed  by  a  consid- 
erable future  increase  of  live  load  and  other  unforeseen  circumstances. 

In  the  Transactions  of  the  American  Society  of  Civil  Engineers,  Vol 
42,  p.  547,  Mr.  H.  S.  Prichard,  discussing  the  provision  for  a  future  increase 
in  live  load  of  100  per  cent.,  states: 

"In  a  good  modern  specification  the  unit  stresses  allowed  are  so 
low  that,  except  for  the  counterstresses,  a  bridge  designed  and  built 
in  accordance  therewith  could  reasonably  be  expected  to  be  able  to 
carry  without  serious  injury  a  live  load  at  least  twice  as  great  as  the 
live  load  specified." 

In  the  Blackwell's  Island  Bridge,  the  impact  of  the  live  load  need  not  impact 
be  considered;  coming  from  three  different  kinds  of  traffic   (rapid  transit.  Negligible 
trolleys  and  wagons)  it  cannot  accumulate,  and  what  little  may  result  is 
absorbed   or   diffused   by  the   solid   floor  and  the  heavy  floor  construction 
before  reaching  the  heavy  trusses;  and,  furthermore,  the  more  the  bridge 
is  crowded,  the  less  motion  of  the  live  load  is  possible. 


32 


CHIEF    ENGINEER'S   REPORT 


But,  assuming,  for  the  sake  of  argument,  that  the  Blackwell's  Island 
Bridge  would  carry  eight  tracks  of  a  steam  railway  with  wooden  ties  instead 
of  with  a  heavy,  solid  floor,  then  the  impact,  according  to  the  specifications 
of  the  American  Railway  Engineering  and  Maintenance  of  Way  Associa- 
tion, would  be  for  the  top  and  bottom  chords  of  the  Island  span  (also  for  the 
previously   mentioned  verticals  U59-L59  and  U73-L73) : 

300  300 


with  continuous  load 


with  discontinuous  load 


8  X  2,796 .+  300        22,668 
300  300 


=  0.013  =  1.3  per  cent, 
=  0.017  =  1.7  per  cent, 


8  X  2,166  + 300  ~  17,628 
since  the  loaded  length  has  to  be  taken  equal  to  the  total  length  of  single 
track;  in  fact,  the  percentage  could  even  be  reduced,  since  the  number  of 
tracks  should  be  increased  to  twelve,  counting  also  the  four  lines  of  wagons 
on  the  roadway. 

Mr.  Prichard,  in  his  paper  "Proportioning  of  Steel  Railway  Bridge  Mem- 
bers" (Proceedings  of  the  Engineers'  Society  of  Western  Pennsylvania,  1907, 
also  "Engineering  News,"  September  19th,  1907),  reduces  the  impact  to 
zero  for  spans  of  1,000  feet  for  one  track,  of  700  feet  for  four  tracks,  and  of 
300  feet  for  eight  tracks. 


Snow  Load 
Negligible 


To  consider  a  snow  load  did  not  seem  necessary.  Assuming  a  thickness 
of  compact  snow  of  12  inches,  or  of  ice  and  slush  of  four  inches,  representing 
about  15  pounds  per  square  foot,  a  larger  amount  may  safely  be  deducted 
from  the  "congested"  live  load  of  100  pounds  per  square  foot  (represent- 
ing a  sohd  mass  of  people  unable  to  move)  or  the  "regular"  hve  load  of  50 
pounds  per  square  foot  (representing  a  crowd  hardly  able  to  move)  of  the 
roadway  or  the  promenades.  To  combine  the  absolute  maxima  of  stresses 
caused  by  a  discontinuous  "congested"  live  load  representing  14  lines  of 
traffic  with  those  caused  by  a  discontinuous  greatest  wind  load  covering  the 
same  discontinuous  stretches  and  no  others  and  do  probably  the  same  thing 
with  a  snow  load  would  be  a  reductio  ad  absurdum. 


Temperature         From  a  uniform  change  of  temperature  the  trusses  of  the  Blackwell's 

stresses  Island  Bridge  receive  stresses  only  on  account  of  the  action  of  the  rocker 

®^'^'   ®  posts  in  the  middle  of  the  river  spans.    When  the  two  adjoining  cantilever 

arms  expand  or  contract,  these  rockers  assume  an  inclined  position,  which 

results  in  the  lowering  of  the  one  and  the  lifting  of  the  other  cantilever  arm, 

causing  insignificant  stresses. 

By  order  of  the  Department  of  Bridges,  early  in  1904,  the  originally 
intended  pin  at  the  bottom  of  each  tower  post  was  replaced  by  a  flat  bear- 
ing; consequently,  the  towers  receive  bending  stresses  from  any  horizontal 
deflection  of  their  top.  From  a  change  of  temperature,  the  top  of  tower 
U75-L75  only  is  deflected,  since  this  is  the  only  tower  where  the  trusses 
are  able  to  slide,  being  rigidly  connected  to  the  piers  at  the  other  three  towers. 
For  a  change  of  +60°  the  deflection  at  the  top  is  3|  inches  and  the  stress 
at  the  bottom  of  tower  3,200  pounds  per  square  inch  in  the  extreme  fiber 


CHIEF   ENGINEER'S   REPORT  33 

of  the  section,  while  the  actual  stress  for  the  dead  and  the  "regular"  con- 
tinuous live  load  is  15,200  pounds  per  square  inch.  The  total  extreme  fiber 
stress,  therefore,  is  only  18,400  pounds  per  square  inch,  which  is  well  below 
20,000  pounds  per  square  inch  permissible  for  a  uniformly  distributed  direct 

stress. 

To  illustrate  the  effect  of  the  "regular"  and  the  "congested"  live  load  Effect  of  ^_ 
of  the  Blackwell's  Island  Bridge  on  an  actually  used  bridge  of  similar  mag-  and  "Con- 
nitude,   the   writer   derived  some  figures  from   a  publication  by   Professor  gested"  Live 
Melan  concerning  the  stiffening  trusses  of  the  Williamsburg  Bridge.    (Hand-  Trusses  of 
buch    der   Ingenieur-wissenschaften,    Arch    and   Suspension    Bridges.)     This  Williamsburg; 
bridge  has  four  trolley  tracks,  only  two  rapid  transit  tracks  (similarly  to     "  ^® 
the  original  plans  for  the  Blackwell's  Island  Bridge),  but  has  two  roadways 
20  feet  each,  two  foot-walks  10  feet  6  inches  each  and  two  "bicycle"  paths 
7  feet  each;  it  has,  therefore,  4  feet  6  inches  wider  roadways,  and  13  feet 
wider  promenades  than  the  Blackwell's  Island  Bridge.    This  would  give  a 
"congested"    load   of    16,000—3,400+450+950=14,000    pounds    per   linear 
foot  of  bridge  and  half  of  this,  that  is,  7,000  pounds  per  linear  foot  of  bridge, 
as   "regular"   load;   but,    assuming  only  those  originally   specified  for  the 
Blackwell's  Island  Bridge  (with  two  rapid  transit  tracks),  namely,   12,600 
and  6,300  respectively,  which  is  10  per  cent,  less,  we  get  for  the  unit  stresses 
in  the  stiffening  trusses  the  following  maxima  in  pounds: 

Top  Chord 

Tension  Compression 

For  "regular"  live  load  alone 21,600  20,800 

For  "regular"  live  load  and  temperature    24,600  23,600 

For  "regular"  live  load,  temperature  and  wind     32,200  30,200 

For  "congested"  live  load  alone 43,200  41,600 

For  "congested"  live  load  and  temperature 46,200  44,400 

For  "congested"  live  load,  temperature  and  wind    ....52,000  50,000 

Bottom   Chord 

For  "regular"  live  load  alone    25,600  19,800 

For  "regular"  live  load  and  temperature    28,200  21,400 

For  "regular"  live  load,  temperature  and  wind 36,000  27,400 

For  "congested"  live  load  alone   51,200  39,600 

For  "congested"  live  load  and  temperature   53,800  40,700 

For  "congested"  live  load,  temperature  and  wind 59,400  46,600 

To  caiise  these  stresses,  the  live  load  has  to  extend  for  only  about  1,100 
feet  in  a  continuous  stretch  from  the  pier,  and  may  be  of  any  length  beyond 
the  pier,  a  condition  which  is  certainly  far  more  possible  than  even  the  con- 
tinuous live  load  mentioned  above  for  the  island  span  of  the  Blackwell's 
Island  Bridge.  These  figures  do  not  take  into  account  any  snow  load  what- 
ever. 

To  treat  the  Williamsburg  Bridge  as  rigorously  as  the  Blackwell's  Island 
Bridge,  it  should  be  considered  that  a  suspension  bridge  has  to  be  so  ad- 
justed that,  for  no  live  load  on  it,  the  ends  of  the  stiffening  trusses  have  no 
reaction  and  that  it  is  very  probable  that  this  condition  is  not  fulfilled  at 


34  CHIEF   ENGINEER'S   REPORT 

the  present  time  in  the  WilUamsburg  Bridge,  which  would  add  some  more 
thousand  pounds  to  the  chord  stresses  in  the  stiffening  trusses. 

To  compare  these  figures  with  those  for  the  Blackwell's  Island  Bridge 
it  should  also  be  kept  in  mind  that  in  a  stiffening  truss  temperature  stresses 
are  positively  sure,  like  dead  load  stresses  in  ordinary  trusses,  since  the  con- 
dition of  no  reaction  of  the  stiffening  trusses  can  be  accomplished  only  for 
a  certain  temperature,  the  stiffening  trusses  being  deflected  and,  therefore, 
stressed  for  any  other  temperature. 

,  Up  to  September  1908,  the  two  elevated  tracks  on  the  Williamsburg 
Bridge  were  not  in  use,  but  the  bridge  had  been  open  to  the  other  traffic 
since  February  1905;  taking  a  "regular"  live  load  of  only  6,300^1,700^ 
4,600  and  a  "congested"  load  of  9,200,  the  above-given  unit  stresses  would 
be  reduced  by  only  about  one-fourth,  would  therefore  be  in  maximum: 

for  the  "regular"   live  load,   29,000  pounds  in  tension,   and  23,000 

pounds  in  compression,  and 

for  the  "congested"  live  load,  44,000  pounds  in  tension,  and  35,000 

pounds  in  compression, 
considerably    greater,    therefore,    than   the    corresponding    values    even    for 
the  whole  traffic  of  8,000  pounds  per  linear  foot  of  bridge  as  "regular"  live 
load,  or  of  16,000  pounds  per  linear  foot  of  bridge  as  "congested"  five  load 
in  discontinuous  stretches  for  the  Blackwell's  Island  Bridge. 

As  a  matter  of  fact,  the  stiffening  trusses  of  the  Williamsburg  Bridge 
were  proportioned  for  a  load  of  4,500  pounds  per  linear  foot  of  bridge,  this 
to  represent  the  whole  traffic  including  the  two  elevated  railroad  tracks; 
no  "congested"  or  extraordinary  load  was  considered. 

Dead  Loads         The  statement  has  been  made  that  the  dead  load  stresses  of  the  Black- 
Compared  well's  Island  Bridge  were  considerably  underestimated.     This  is  only  partly 

with  Subse-  correct.  The  designers  realized  from  the  beginning  that  the  dead  load  of 
AssumpUons  ^  highway  bridge,  especially  of  one  of  this  magnitude,  is  of  the  greatest 
importance.  For  this  reason,  the  floor-beams  were  curved  in  their  top  chord, 
to  follow  the  crowning  of  the  paving  in  order  to  reduce  its  weight  to  a  mini- 
mum. There  are  city  bridges  in  existence  for  which  the  weight  of  the  paving 
was  assumed  for  the  design  with  75  pounds  per  square  foot,  while  it  actually 
amounts  to  150  pounds,  and  even  more,  eating  up  all  allowance  for  live  load. 
The  Department  estimated  the  weight  of  the  paving  material  for  the  lower 
floor,  concrete  at  40  pounds,  wood  blocks  at  20  pounds,  and  for  the  prome- 
nades at  20  pounds  per  square  foot. 

The  original  estimate  of  the  steel  weight,  made  by  the  Department  of 
Bridges,  was  86,000,000  pounds.  Beginning  early  in  1904,  the  Department 
ordered  changes  and  additions  made,  such  as  changing  the  tracks  of  the 
upper  floor  from  longitudinal  ties  with  bulb  angles  to  wooden  cross  ties, 
addition  of  two  elevated  railroad  tracks,  reduction  of  pressure  of  the  tower 
bases  on  the  masonry,  replacing  the  pin-bearing  of  the  bottom  of  the  towers 
by  a  flat  bearing,  additional  sway  bracing,  bottom  and  top  laterals,  top 
struts,  bracing  between  stringers,  secondary  verticals  from  the  middle  pins 


CHIEF   ENGINEER'S   REPORT  35 

to  the  top  chord,  etc.  (See  "Report  of  the  Commissioner  of  the  Department 
of  Bridges,  1904,"  printed  in  1905,  pages  31,  32,  33,  34),  all  this  increas- 
ing the  steel  weight  to  about  95,000,000  pounds,  with  a  total  dead  load  of 
116,000,000  pounds.  New  dead  load  stress  sheets  were  made  by  the  Depart- 
ment, and  at  their  suggestion,  also,  by  the  contractor  subject  to  their  ap- 
proval, in  order  to  hasten  the  time  of  the  ordering  of  the  material.  To  these 
dead  load  stresses,  the  live  load  stresses  calculated  by  the  Department  for 
the  new  live  load  (with  four  elevated  railroad  tracks)  were  added,  the  sec- 
tions of  the  truss  members  determined  and  all  section  sheets  which  the  con- 
tractor prepared  for  use  in  his  own  drawing-room  were  submitted  to  and 
finally  approved  by  the  Department  with  certain  changes. 

Shop  drawings  for  the  Island  span  were  now  made  by  the  contractor 
and  approved  by  the  Department  with  changes,  and  the  material  ordered. 
In  the  meantime,  new  dead  load  concentrations  and  new  stress  sheets  were 
worked  out,  finished  about  November,  1904,  based  on  a  new  estimate  of 
the  steel  weight  of  100,750,000  pounds,  giving  with  the  originally  prescribed 
weight  of  paving,  pipes,  etc.,  a  total  dead  load  of  122,130,000  pounds.  As 
far  as  possible,  the  old  stress  and  section  sheets  were  then  revised.  To  change 
the  material  for  the  Island  span,  at  that  time  in  the  process  of  manufac- 
ture in  the  Bridge  Shop,  was  impossible,  and  it  is  here  that  some  dead  load 
stresses  figured  with  these  new  dead  load  concentrations  slightly  overrun 
the  older  figures.  The  actual  shipping  weight  of  the  bridge  proper  (deduct- 
ing test  eye-bars  and  other  test  material)  is  about  105,150,000  pounds,  which, 
of  course,  includes  all  changes  and  additions  made  after  November,  1904, 
as  reinforcing  certain  trolley  stringers  for  elevated  railroad  trains,  strength- 
ening of  the  anchorages,  etc.,  giving  a  total  dead  load  of  125,680,000  pounds. 
It  should  be  stated  in  this  connection  that  certain  changes  in  the  design, 
which  caused  a  considerable  addition  to  the  steel  weight,  did  not  affect  the 
dead  load  stresses,  as  strengthening  of  the  anchorages,  increase  in  weight 
of  tower  bases  and  towers,  in  the  weight  of  the  box  girders  between  trusses 
at  the  piers,  in  the  weight  of  the  shoes  on  top  of  the  towers,  etc. 

About  a  year  ago,  after  the  whole  bridge  material  had  been  fabricated, 
the  writer,  for  his  own  satisfaction,  had  new  stress  sheets  prepared,  using 
the  dead  load  concentrations  of  November,  1904,  and  the  stresses  from 
the  continuous  "regular"  Uve  load  given  us  by  the  Department  of  Bridges, 
considering  also  the  fiber  stresses  due  to  the  own  weight  of  the  truss  members, 
and  satisfied  his  own  mind  that  the  bridge  was  able  to  carry  the  specified 
traffic.  The  dead  load  concentrations  of  November,  1904  were  considered 
sufficiently  accurate,  as  they  were  based  on  a  total  dead  load  of  122,130,000 
pounds,  while  the  actual  dead  load  using  the  shipped  steel  weight  was  only 
3  per  cent,  higher,  that  is,  125,680,000. 

In  the  meantime,  the  Department  changed  the  plans  for  the  paving. 
As  mentioned  above,  the  original  paving,  consisting  of  concrete  and  wooden 
blocks,  was  intended  to  be  as  light  as  possible  and  to  extend  only  between 
the  curbs  for  35^  feet;  the  tee-rails  for  the  inside  trolley  tracks  were  to  be 
5  inches  high  and  their  top  about  6  inches  above  top  of  stringer;  the  buckle 


36  CHIEF   ENGINEER'S   REPORT 

plates  between  the  rails  merely  covered  with  asphalt  and  the  rails  free.  The 
Department  now  decided  to  extend  the  paving  over  the  whole  roadway 
between  the  trusses  (53  feet)  with  heavy  cast-iron  curbs  and  of  much  greater 
thickness,  using  7-inch  grooved  rails  with  their  tops  nearly  12  inches  above 
the  top  of  the  stringer,  and  imbedded  entirely  in  concrete.  If  ordinary  con- 
crete and  not  cinder  concrete  be  used,  this  makes  the  paving  about  1,341 
pounds  per  linear  foot  heavier  than  originally  intended  and  assumed  in  the 
calculations,  increasing  the  total  dead  load  to  132,300,000  pounds,  which 
figure  was  used  by  the  experts  in  their  calculations  of  the  dead  load  stresses. 
The  change  in  the  paving  was  an  after-thought,  not  concerning  the  con- 
tractor, since  at  that  time  the  bridge  was  completely  manufactured,  and, 
in  its  greatest  part,  erected.  This,  and  the  fact  that  the  additional  paving, 
being  of  uniform  weight  over  the  whole  bridge,  forms  a  larger  percentage 
of  the  dead  load  near  the  ends  of  the  cantilever  arms,  where  its  influence 
on  the  stresses  is  the  greatest,  accounts  for  the  great  difference  in  the  dead 
load  stresses  as  used  for  the  design  of  the  bridge  and  as  calculated  by  Pro- 
fessor Burr  and  Messrs.  Boiler  &  Hodge. 

The  writer  understands  that  the  Department  of  Bridges  recalculated 
the  dead  load  stresses  after  arranging  for  heavier  paving,  and  that  they  got 
the  same  excessive  figures  as  the  experts,  but  that  they  had  not  come  to  any 
conclusion  pending  a  recalculation  of  the  live  load  stresses  when  the  experts 
were  invited  to  make  a  report. 

Although  it  does  not  seem  within  the  scope  of  this  report,  it  may  never- 
theless be  useful  to  mention,  for  the  information  of  those  critics  of  the  Black- 
well's  Island  Bridge  who  are  not  experts  in  bridge  engineering,  that  the 
excellence  of  the  design  of  different  bridges  cannot  be  judged  by  the  propor- 
tion of  the  steel  weight  necessary  to  carry  a  pound  of  live  load.  This  depends 
for  bridges  of  similar  character  almost  entirely  on  the  length  of  the  span. 
For  a  very  short  span,  one  pound  of  steel  may  suffice  to  carry  one  hundred 
pounds  of  live  load,  and  for  a  very  long  span  the  reverse  may  be  the  case. 
The  proportion  of  steel  weight  to  the  assumed  working  live  load  is  for  the 
following  bridges  approximately: 

Greatest 
Span       Proportion 

Thebes  bridge 671'  1.0 

Monongahela  bridge 812'  1.0 

Memphis  bridge  (one  track  only)    790'  1.7 

Blackweli's  Island  bridge  (heavy  solid  floor)    1,182'  3.4 

Quebec  bridge     1,800'  4.3 

Firth  of  Forth  bridge    1,710'  4.7 

The  Blackweli's  Island  Bridge  carries  a  heavy  solid  floor,  the  others  only 
wooden  ties.  If  the  former  had  had  a  fight  wooden  floor,  possibly  2.5  pounds 
of  steel  instead  of  3.4  pounds  would  have  been  sufficient  to  carry  a  pound  of 
live  load. 


CHIEF    ENGINEER'S   REPORT  37 

The  report  of  Messrs.  Boiler  &  Hodge  states:  Reverse  ajid 

Secondary 
"We  have  made  no  additions  for  reverse  stresses,  as  the  speci-  stresses 

fications  state  that  the  sections  are  to  be  computed  for  the  stress 
requiring  the  greatest  area,  so  that  the  unit  stresses  we  have  shown 
are  the  direct  stresses  from  dead  and  live  loads  without  any  addi- 
tions for  reverse  stresses,  snow,  wind,  impact,  or  secondary  stresses." 

The  effect  of  snow,  wind  and  impact  has  been  mentioned  before;  there 
remains  now  to  discuss  reverse  and  secondary  stresses. 

In  most  specifications  for  railroad  bridges  written  in  the  last  ten  years, 
the  effect  of  a  reversal  of  stress  is  only  considered  if  the  reversal  occurs  in 
immediate  succession,  as  "counter  stresses  in  web  members  or  chords  in 
continuous  trusses"  (this  is  the  usual  wording)  under  the  passage  of  the 
same  train;  in  highway  bridges,  it  is  as  a  rule  disregarded.  It  is  hard  to  con- 
ceive how  a  reversal  of  stress  could  happen  in  the  Blackwell's  Island  Bridge 
in  immediate  succession  with  the  different  kinds  of  live  load. 

The  report  of  Messrs.  Boiler  &  Hodge  states: 

"The  secondary  stresses  due  to  distortion  of  the  true  figures  of 
the  trusses  by  the  live  load  are  quite  considerable,  as  the  vertical 
deflection  of  the  point  L37  is  18-j^  inches,  and  of  the  point  L91,  14-j^q 
inches  for  a  live  load  of  3,000  pounds  per  1  linear  foot  of  truss. 

"  We  have  made  a  careful  analytical  computation  of  the  hori- 
zontal movement  of  the  point  U17  (and  other  similar  points  over  the 
main  piers)  caused  by  this  distortion. " 

To  avoid  possible  misunderstanding,  it  may  be  well  to  point  out  that 
the  horizontal  movement  of  point  U17  (top  of  tower)  is  not  caused  by  the 
vertical  deflection  of  point  U37  (where  the  cantilever  arms  join),  but  merely 
by  one  contributing  cause  to  this  vertical  deflection,  namely  the  deformation 
of  the  anchor  arm.,  the  other  more  important  cause  contributing  to  the 
vertical  deflection  of  point  U37  being  the  deformation  of  the  cantilever  itself 
which  does  not  affect  the  horizontal  movement  of  the  top  of  the  tower. 

The  effect  of  secondary  stresses,  which  lately  has  been  given  its  deserved 
prominence,  should,  however,  not  be  over-estimated.  In  the  case  of  rolled 
or  riveted  girders,  in  bending,  we  allow  the  same  extreme  fiber  stresses  as  for 
axial  stresses,  since  the  chords  are  concentrated  as  in  an  articulated  truss, 
being  kept  apart  by  the  web.  In  bending  of  a  compact  section,  however, 
like  a  pin,  we  allow  up  to  50  per  cent,  more  stress,  which  is  not  only  based 
on  experience  with  pins  but  also  on  theory,  or  rather  the  non-fulfilment  of 
one  assumption  of  the  theory  of  bending,  viz.,  that  the  fibers  deform  inde- 
pendently. The  fiber  stresses  not  being  uniformly  distributed  over  the  whole 
section,  only  the  outer  rows  having  the  maximum  stress,  the  less-stressed 
adjoining  fibers  must  hold  in  position,  that  is,  relieve,  the  overstressed  fibers. 
The  assumption  that  a  cross  section  remains  a  plane  after  bending  is,  in 
reality,  not  strictly  fulfilled,  even  for  stresses  within  the  elastic  limit,  and 


38  CHIEF   ENGINEER'S   REPORT 

the  beam  is  actually  stronger  than  would  seem  from  the  theory  of  bending. 
The  allowable  bending  stress  should  be  derived  from  bending  tests,  but  the 
results  would  vary  with  the  shape  of  the  cross  section. 

The  secondary  stresses  caused  by  the  own  weight  of  the  members  and 
by  the  distortion  of  the  truss  are  bending  stresses,  and  as  such  could  be 
allowed  for  the  compact  sections  of  the  truss  members  of  the  Blackwell's 
Island  Bridge  at  least  20  per  cent,  higher  units  than  for  axial  tension.  Cooper's 
standard  specifications  (and  others)  take  this  into  account,  specifying  "if 
the  fiber  strain  resulting  from  the  weight  only  of  any  member  exceeds  10  per 
cent,  of  the  allowed  unit  strain  on  such  member,  such  excess  must  be  con- 
sidered in  proportioning  the  area."  Most  specifications  disregard  these 
stresses  entirely. 

The  original  specifications  (1903)  did  not  contain  a  clause  to  consider 
these  fiber  stresses.  In  April,  1904,  the  Department  decided  to  take  them 
into  account,  this  to  the  writer's  recollection  having  been  done  in  order  to 
be  on  the  safe  side  in  proportioning  the  truss  members  liberally,  as  it  was 
realized  at  that  time  that  the  actual  dead  load  stresses  may  easily  overrun. 

The  Quebec         The  writer  is  not  aware  that  the  failure  of  the  Quebec  Bridge  caused 
and  the  ^ny  "suspicion"   (as  stated  in  a  technical  paper  a  few  weeks  ago)  of  the 
Blackwell's  safety  of  the  Blackwell's  Island  Bridge.    He  does  not  think  that  such  was 
the  case  until  the  Royal  Canadian  Commission,  which  was  appointed  "to 
inquire  into  the  cause  of  the  collapse  of  the  Quebec  Bridge,"  made  the  follow- 
ing published  statements: 

"By  reference  to  the  table,  it  will  be  seen  that  the  specified  stresses 
for  the  Quebec  Bridge,  under  working  conditions,  are  in  advance 
of  current  practice,  and  we  believe  they  are  without  precedent  in 
the  history  of  bridge-engineering.  Under  extreme  conditions,  the 
Quebec  Bridge  stresses  are  in  general  harmony  with  those  permitted 
in  the  Blackwell's  Island  Bridge."    (Page  148  of  their  report.) 

And,  comparing  the  bottom  chords  of  the  two  bridges  (page  140): 

"The  development  of  the  detail  plans  of  the  Blackwell's  Island 
Bridge  was  contemporaneous  with  that  of  the  Quebec  Bridge  plans; 
the  Quebec  designers  had  not  access  to  the  Blackwell's  Island  plans. 
In  fairness  to  the  Quebec  Bridge  designers,  however,  it  should  be 
pointed  out  that  in  the  Blackwell's  Island  Bridge  the  proportions  of 
many  of  the  details  are  much  more  nearly  in  accord  with  Quebec 
Bridge  practice  than  are  those  of  the  earlier  bridges,  although  the 
principles  of  the  designs  are  very  different." 

Now,  the  Blackwell's  Island  Bridge  was  not  on  trial  and,  being  unfinished, 
a  comparison  of  unit  stresses  allowed  for  the  sum  of  the  dead  and  imaginary 
live  loads  did  not  prove  anything  as  far  as  the  Quebec  failure  was  concerned, 
merely  creating  a  false  impression.  It  would  not  even  have  proven  anything 
if  the  Blackwell's  Island  Bridge  had  been  in  use,  since  the  Quebec  Bridge 
was  a  steam  railway  bridge,  designed  to  carry  two  tracks  of  comparatively 


CHIEF   ENGINEER'S   REPORT  39 

light  loading,  and  neglecting  entirely  any  live  load  on  the  two  roadways 
of  17  feet  width  each,  while  the  Blackwell's  Island  Bridge  was  designed 
to  carry  eight  tracks  for  electric  traffic,  which  were  assumed  loaded  with  a 
heavy  load  in  addition  to  a  heavy  load  on  the  roadways  and  promenades,  a 
total  live  load  unprecedented  for  any  highway  bridge,  even  in  New  York  City. 

The  Commission  compares  the  six  largest  cantilever  bridges  as  to  the 
live  loads  and  permissible  unit  stresses  assumed  when  they  were  designed. 
It  would  have  been  more  interesting  to  have  indicated  the  greatest  live 
loads  actually  used  at  the  present  time  on  the  four  which  are  in  use. 

The  Forth  Bridge  (designer  Sir  Benjamin  Baker)  is  designed  for  two 
tracks,  each  loaded  with  approximately  Cooper's  E22  loading,  and  a  com- 
pression allowed  of  17,000  pounds  per  square  inch,  with  no  reduction  for 
buckling,  and  16,350  pounds  per  square  inch  for  tension;  no  impact  is  con- 
sidered. The  actual  live  load  on  this  bridge  has  certainly  increased  well 
beyond  the  original  assumptions,  and  may  have  to  be  doubled  and  tripled 
in  the  near  future.  (In  this  country,  the  Atchison,  Topeka  &  Santa  Fe  use 
Cooper's  E66,  while  the  New  York,  New  Haven,  the  Carolina,  Clinchfield 
&  Ohio,  the  Lake  Erie  &  Western,  the  Crescent  line,  the  Tidewater  R.  R., 
specify  a  working  live  load  of  Cooper's  EGO.) 

The  Monongahela  Bridge  (designers  Messrs.  Boiler  &  Hodge)  is  designed 
for  two  tracks,  each  for  E45  working  load,  and  considering  100  per  cent, 
impact  to  the  live  load,  allows  21,000  pounds  per  square  inch  compression, 
with  no  reduction  for  l/r  smaller  than  40,  and  22,000  pounds  per  square  inch 
for  tension. 

The  Thebes  Bridge  (designer  Mr.  R.  Modjeski)  is  designed  for  two  tracks, 
each  for  a  working  load  of  a  little  less  than  E50,  and,  considering  100  per 
cent,  impact  to  the  live  load,  allows  21,000  pounds  per  square  inch  compres- 
sion, with  no  reduction  for  l/r  smaller  than  about  45,  and  a  tension  of  20,000 
pounds  per  square  inch. 

The  Memphis  Bridge  (designer  Mr.  G.  S.  Morison)  is  designed  for  one 
track  for  a  loading  of  E40,  allowing  a  compression  of  14,000  pounds  per  square 
inch,  with  no  reduction  for  l/r  smaller  than  45,  and  no  consideration  of  impact; 
and  a  tension  of  20,000  pounds  per  square  inch,  adding  an  impact  of  100 
per  cent,  to  the  live  load. 

The  Quebec  Bridge  was  designed  for  two  tracks,  carrying 

(1)  A  working  load  for  certain  members  hardly  equal  to  E30,  with  a 
permissible  unit  tension  and  compression  reaching  21,200  pounds  per  square 
inch  (f.  i.  in  the  compression  chords)  with  impact  considered  by  means  of 
a  "minimum  over  maximum"  formula,  but  no  reduction  for  buckling  made 
for  l/r  smaller  than  50  (no  reduction,  therefore,  for  the  compression  chords) ;  or, 

(2)  An  extraordinary  load  of  50  per  cent,  more,  therefore,  about  E45, 
with  a  permissible  unit  stress  of  24,000  pounds  per  square  inch  in  tension, 
and  also  in  compression  for  the  chords  and  main  diagonals,  and  of  24,000 — 
100  l/r  in  compression  for  the  posts.  (A  snow  load  of  1,600  pounds  per  linear 
foot  of  bridge  was  added  to  the  specifications  in  June,  1905,  after  the  stress 
sheets  and  some  shop  drawings  were  approved.) 


40  CHIEF   ENGINEER'S    REPORT 

In  order  to  compare  the  "safety"  of  these  five  bridges,  we  would  not  only 
have  to  compare  the  unit  stresses,  the  assumed  live  loads,  the  loaded  length 
causing  maximum  stresses,  the  assumed  impacts,  etc.,  but  also  investigate 
how  closely  the  assumed  live  loads  approach  the  present  everyday  work- 
ing loads,  and  the  possibiUty  of  a  future  increase  in  the  latter  for  which, 
with  the  exception  of  the  Quebec  Bridge,  no  provision  was  made,  no  extra- 
ordinary load  having  been  specified  with  corresponding  increase  in  unit 
stresses. 

The  comparison  of  the  design  of  these  structures,  all  of  them  being  rail- 
road bridges,  is  therefore,  not  a  simple  matter;  how  much  more  complex 
to  compare  them  with  the  Blackwell's  Island  Bridge,  which  was  designed 
for  a  h'ghway  traffic  of  unprecedented  complexity. 

In  a  double-track  railroad  bridge,  the  trusses  are  stressed  close  to  the 
permissible  maximum  every  time  two  trains  meet  on  the  br  dge,  and  yet 
most  engineers  allow  higher  unit  stresses  than  for  single-track  bridges,  or 
reduce  the  five  load  per  track. 

Sir  Benjamin  Baker,  in  a  paper  on  "Working  Stress  of  Iron  and  Steel," 
read  before  the  American  Society  of  Mechanical  Engineers  in  1886,  remarks 
justly  (see  Railroad  Gazette,  February  18th,  1887):  "A  machine  or  bridge 
can  only  be  well  proportioned  by  carefully  considering  the  special  condi- 
tions of  the  case,  in  the  light  of  experimental  data  and  past  experience.  A 
string  of  formulae  will  not  make  an  engineer." 

In  the  report  of  the  Royal  Commission  on  the  Quebec  Bridge,  page  161, 
are  given  the  specifications  revised  by  Mr.  Theodore  Cooper  in  1904  as  fol- 
lows: 

First  case.— "The  maximum  strains  produced  by  the  following 
live  loads  and  wind  shall  be  used  for  proportioning  all  members  of 
the  trusses  or  towers: 

/'(I)   A  continuous  train  of  any  length,   weighing  3,000   pounds 
per  foot  of  track,  moving  in  either  direction  on  each  track. 

"  (2)  A  train  900  feet  long,  consisting  of  two  E33  engines,  followed 
by  a  load  of  3,300  pounds  per  linear  foot,  upon  each  railroad  track 
and  moving  in  either  direction. 

"  (3)  A  train  550  feet  long,  consisting  of  one  E40  engine,  followed 
by  4,000  pounds  per  linear  foot  of  track,  on  each  track. 

"  (4)  For  the  suspended  span  a  lateral  wind  force  of  700  pounds 
per  linear  foot  of  the  top  chord  and  1,700  pounds  per  linear  foot  of 
the  lower  chord,  one-half  of  which  shall  be  used  for  lateral  and  diag- 
onal bracing. 

"For  the  cantilever  and  anchor  arms,  a  lateral  force  of  500  pounds 
on  the  top  chord  and  1,000  pounds  on  the  lower  chord,  per  Unear  foot, 
in  addition  to  the  wind  force  on  the  suspended  span,  shall  be  con- 
sidered. 

"Only  one-third  of  this  maximum  wind  force  need  be  considered 
in  proportioning  the  chords.    It  shall  be  considered  as  a  live  load. 


CHIEF    ENGINEER'S   REPORT  41 

Unless  this  increases  the  strains  due  to  the  Uve  and  dead  loads  only 
more  than  25  per  cent,  the  sections  need  not  be  increased." 

Second  case. — On  page  162,  provision  for  a  future  increase  of  50 
per  cent,  in  the  train  loads  is  called  for. 

Mr.  C.  C.  Schneider,  who  was  appointed  on  behalf  of  the  Canadian  Gov- 
ernment "to  inquire  into  and  pass  upon  the  sufficiency  of  the  present  design 
of  the  Quebec  Bridge,"  independently  of  the  Royal  Commissioners,  states 
as  follows: 

"The  first  case  will  be  called  hereafter  the  working  load,  and 
the  second  case  the  extreme  load.  The  strains  produced  by  the  work- 
ing load,  which  is  by  no  means  excessive,  should  leave  a  reasonable 
margin  for  safety.  The  strains  produced  by  the  extreme  loads  should 
remain  within  the  elastic  limit  of  the  material." 
On  page  156,  Mr.  Schneider  reports  as  follows: 

"The  extreme  unit  strains  within  which  in  the  writer's  judgment 
the  structure  may  be  considered  to  be  able  to  sustain  the  loads  pro- 
vided for  in  the  specifications,  are: 

(1)  For  the  dead  and  live  loads  combined  with  the  snow  load: 
For  tension,  21,000  pounds  per  square  inch  of  net  section;  for  com- 
pression, 21,000 — 90  l/r  per  square  inch  of  gross  section. 

"  (2)  For  the  extreme  provisions  of  one  and  one-half  times  the 
live  load,  dead  and  snow  loads,  combined  with  one-third  of  the  wind 
strains:  For  tension,  24,000  pounds  per  square  inch  of  net  section, 
for  compression,  24,000 — 100  l/r  per  square  inch  of  gross  section." 

From  this  follows  that  Mr.  Schneider  considered  for  the  Quebec  Bridge 
a  live  load  (from  less  than  E30  to  less  than  E40  for  the  different  truss  mem- 
bers) which  in  all  probability  would  have  occurred  every  time  two  trains 
had  met  on  the  center  span  as  "working  load,"  and  a  live  load  50  per  cent, 
higher  as  "extreme  load,"  and  for  these  loads  together  with  the  dead  and 
specified  snow  loads  he  assumes,  but  "does  not  advocate,"  unit  stresses  not 
exceeding  21,000  and  24,000  pounds  respectively.  These  live  loads  cannot 
be  compared  with  those  of  the  Blackwell's  Island  Bridge,  that  is  8,000  pounds 
per  linear  foot  of  bridge  as  "regular"  and  16,000  pounds  per  linear  foot 
of  bridge  as  "congested." 

It  should  also  be  remembered  that  the  Quebec  Bridge  was  built  to  carry 
besides  the  double-track  steam  railway  also  "a  roadway  on  each  side  17 
feet  wide  in  the  clear,  suitable  for  ordinary  highway  traffic  with  one  electric 
railway  track  on  each  roadway,"  and  that  for  the  roadways  absolutely  no 
allowance  was  made  in  the  live  load  for  the  trusses,  originally  not  even  for 
a  snow  load,  so  that  at  least  the  effect  of  a  snow  load  should  be  added  to  the 
originally  specified  unit  stresses  before  a  comparison  is  made.  In  the  Black- 
well's  Island  Bridge  a  snow  load  can  safely  be  deducted  from  the  assumed 
uniformly  distributed  live  load  on  the  roadways  and  footwalks,  since  it  is 
only  reasonable  to  assume  that  the  live  load  will  be  reduced  by  that  amount. 


42  CHIEF   ENGINEER'S   REPORT 

Considering  the  probabilities  of  the  different  live  loads  specified  for  the 
Quebec  Bridge  and  the  Blackwell's  Island  Bridge,  it  would  seem  more  correct 
to  compare  the  effect  of  the  "extreme"  load  plus  the  snow  load  on  the  Que- 
bec Bridge  with  the  effect  of  the  "regular"  load  of  8,000  pounds  per  linear 
foot  of  bridge,  in  continuous  stretches  on  the  Blackwell's  Island  Bridge. 
This  would  give,  for  instance,  for  the  bottom  chords  a  greatest  unit  stress 
in  compression  of  30,100  pounds  for  the  Quebec  Bridge,  to  be  compared 
with  17,000  pounds  for  the  Blackwell's  Island  Bridge. 

The  following  greatest  unit  stresses  in  compression  for  the  bottom  chords 
may  be  of  interest: 

Quebec  Bridge 

For  "extreme"  condition,  after  completion  .  .  .  .30,100  (24,000  specified) 
For  "working"  condition,  after  completion  .  .  .  .24,600  (20,500  specified) 
For  dead  load  alone  after  completion 17,500 

Blackwell's  Island  Bridge 

With  a  live  load  of  16,000  pounds  per  linear  foot 

after  completion    22,100  (21,300  specified) 

With  a  live  load  of  8,000  pounds  per  linear  foot 

after  completion 17,000  (17,600  specified) 

For  dead  load  alone  after  completion 11,900 

To  illustrate  further  Mr.  Schneider's  ideas  of  the  difference  between 
"working"  and  "extreme"  load,  the  following  taken  from  his  closing  dis- 
cussion of  his  paper  on  "The  Structural  Design  of  Buildings"  (Transactions 
of  the  American  Society  of  Civil  Engineers,  1905)  may  be  of  interest: 

"Some  of  the  discussors  have  discovered  that  it  is  possible  to  obtain 
greater  loads  than  those  specified.  The  writer  wishes  to  state  that 
he  was  fully  aware  of  all  these  possibilities  of  loading,  and,  also,  has 
carefully  studied  all  the  literature  on  the  subject  of  experiments  on 
the  weights  of  crowds  of  people,  .  .  .  and  has  given  all  these  facts 
due  consideration  in  determining  the  live  loads  to  be  specified  for  build- 
ings. .  .  .  The  writer,  in  determining  upon  the  live  loads  of  build- 
ings of  various  kinds,  as  stated  before,  has  been  guided  by  what  is 
now  considered  the  best  practice  in  bridge-building.  It  is  not  a  ques- 
tion of  how  much  load  of  any  kind  it  is  possible  to  pile  on  a  square 
foot  of  floor  area.  The  question  is  to  make  the  buildings  absolutely 
safe  without  wasting  large  quantities  of  material  in  places  where  it  is 
not  needed.  A  structure  should  be  proportioned  for  a  working  load 
with  the  ordinary  unit  strains,  and  provision  made  for  a  congested 
load  with  unit  strains  well  within  the  elastic  limit,  so  that  the 
structure  may  yet  be  safe  under  such  extraordinary  conditions  of  load- 
ing. The  working  load  should  be  the  probable  maximum  load  which 
may  be  reasonably  expected  to  occur,  while  the  congested  load  is  a 
load  which  is  improbable,  but  within  the  reach  of  possibility." — 


CHIEF    ENGINEER'S   REPORT  43 

"Railroad  bridges  are  now  generally  designed  for  a  possible  future 
increase  in  the  weight  of  locomotives  and  rolling  stock,  but  no  engi- 
neer would  think  of  designing  a  long-span,  double-track  bridge  under 
the  assumption  that  both  tracks  will  be  entirely  covered  with  the 
heaviest  type  of  locomotives,  or  cars  carrying  heavy  ordnance  and 
using  the  ordinary  unit  strain  for  that  condition. 

"The  writer  desires  to  emphasize  the  fact  that  such  unusual  and 
extraordinary  conditions  of  loading  have  been  considered  in  his  speci- 
fications, in  assuming  that  it  is  rational  and  unquestionably  good 
practice  to  allow  a  unit  strain  of  25  per  cent,  in  excess  of  the  ordinary 
working  strain,  or  20,000  pounds  per  square  inch  for  congested  loads 
and  50  per  cent,  in  excess,  or  24,000  pounds  per  square  inch  for  very 
extreme  cases. 

"There  are  railroad  bridges  in  existence  now,  some  members 
of  which  are  strained  to  24,000  pounds  per  square  inch,  including 
impact,  almost  every  time  a  train  passes." 

The  statement  has  been  made  that,  in  the  last  years,  the  permissible  High  Unit 
unit  stresses  used  in  the  design  of  bridges  have  been  increased;  and  "high"   Desfm^of" 
permissible  unit  stresses  and  our  "ignorance"  in  regard  to  the  proper  design  Compression 
of  compression  members  were  widely  criticized  ever  since  the  Quebec  Bridge      ®™  *" 
failed. 

The  writer,  based  on  his  own  investigations,  is  of  the  opinion  that  if  the 
design  of  some  of  the  older  bridges  as  described  in  the  Transactions  of  the 
American  Society  of  Civil  Engineers  and  other  publications  would  be  ana- 
lyzed, especially  with  the  actual  and  not  the  assumed  dead  loads  and  the 
assumed  light  live  loads  properly  considered,  the  statement  concerning 
unit  stresses  would  appear  in  a  somewhat  different  light.  The  impression 
was  probably  caused  by  a  few  specifications  allowing  high  unit  stresses 
for  a  practically  impossible  excessive  live  load  in  order  to  provide  amply 
for  counterstresses  in  tension  members  which  might  be  caused  by  a  possi- 
ble increase  of  the  ordinary  working  load  and  other  unforeseen  circumstances, 
without  increasing  unduly  the  costs  of  the  bridge.  This  precaution  may  be 
of  great  value  (as  mentioned  before)  while  the  additional  weight  is  only 
small. 

Sir  Benjamin  Baker  in  the  above-named  paper  (in  1886)  states  as  follows: 

"The  writer  has  availed  himself  of  the  opportunity  afforded  by 
the  large  use  of  special  plant  and  machinery  at  the  Forth  Bridge 
works  to  note  the  influence  of  varying  stresses  on  full-sized  riveted 
steel  girders.  These  observations  are  still  in  progress  and  can  be 
but  very  briefly  referred  to  herein.  In  one  instance  the  lever  of  a  large 
plate-bending  press  is  of  box-girder  section,  built  up  of  eight  4  x  4  x  f 
inch  angle  bars,  13  x  |  inch  web  plates,  and  two  17  x  ^  x,f  inch  flanges. 
The  span  is  15  feet  8  inches,  and  the  ordinary  daily  working  stress 
on  the  metal  is  43,000  pounds,  and  occasionally  57,000  pounds  per 


44  CHIEF   ENGINEER'S   REPORT 

square  inch.  Many  thousand  applications  of  this  stress  have  been 
made,  and  the  beam  has  taken  a  permanent  set  of  seven-eighths  of 
an  inch,  but  so  far  is  otherwise  intact." — 

"As  regards  the  important  question  of  the  proper  working  stress 
on  iron  and  steel,  the  writer's  experience  leads  him  to  believe  that 
both  the  old-fashioned  government  regulations,  giving  the  same 
limiting  stress  for  all  kinds  of  loading,  and  the  modern  formula,  based 
chiefly  on  Wohler's  experiments,  fail  to  meet  the  just  requirements 
of  the  practical  engineer.  It  is,  in  many  cases,  a  great  economical 
advantage  and  convenience  to  have  reference  not  merely  to  the  vari- 
ation of  stress,  but  also  to  the  probable  number  of  applications.  For 
example,  the  writer  knows  the  bending  press  box  girder  lever,  previ- 
ously referred  to,  will  last  its  time,  although  the  working  stress  is 
about  two-thirds  of  the  ultimate  strength  of  the  material;  and  it 
would  have  been  a  mere  waste  of  money  to  make  it  four  times  as 
strong,  and  so  give  it  the  factor  of  safety  of  six,  usual  and  proper 
enough  for  a  structure  such  as  the  Elevated  Railway  of  New  York, 
where  a  practical  infinite  number  of  repetitions  of  stress  have  to  be 
provided  for." — 

"The  Conway  Tubular  Bridge,  which  has  carried  the  heavy  traf- 
fic of  the  London  &  Northwestern  Railway  for  the  past  thirty-six 
years,  is  412  feet  in  span,  and  under  its  own  weight  the  tensile  stress 
is  13,000  pounds  per  square  inch.  With  ordinary  trains,  the  stress 
is  17,000  pounds,  and,  if  covered  with  the  heaviest  engines  in  use 
on  the  line,  20,000  pounds  per  square  inch.  The  ultimate  strength 
of  the  riveted  structure  is  about  42,000  pounds  per  square  inch.  No 
indications  of  weakness  have  developed  during  the  thirty-six  years' 
working  nor  anything  to  suggest  that  the  factor  of  safety  of,  say 
2  to  2^,  is  unduly  low." — 

"Twenty  years  ago,  being  uncontrolled  by  government  regula- 
tions, the  writer  adopted  a  working  stress  of  16,000  pounds  per  square 
inch*  on  many  large  iron  girders  carrying  a  heavy  dead  load,  although 
at  that  time  a  departure  from  the  usual  11,200  pounds  per  square 
inch  was  regarded  with  suspicion.  The  results  of  modern  research 
have,  however,  now  given  the  engineer  a  free  hand,  and  the  British 
five  tons  per  square  inch  and  the  Continental  six  kilos  per  square 
millimeter  have  ceased  to  be  regarded  with  superstitious  reverence." 

Professor  Engesser,  in  his  well-known  book  on  secondary  stresses,  pub- 
lished in  1893,  writes  as  follows: 

"For  repeated  stresses,  a  rupture  of  the  material  takes  place 
for  stresses  below  the  ultimate  strength,  depending  on  the  range 
of  the  stress.  This  ultimate  for  repeated  stresses  is  the  lower,  the 
greater  the  range,  and  has  its  lowest  value  for  alternate  tension  and 

♦This  refers  to  working  live  load  without  impact  and  to  iron,  not  steel.  The  writer  was  unable  to  find 
any  heavier  bridge  built  by  Mr.  Baker  at  that  time  than  the  Mersey  bridge,  built  1869,  near  Liverpool,  a 
span  of  305  feet  with  a  weight  of  steel  probably  only  three-fourths  of  the  assumed  live  load. 


CHIEF   ENGINEER'S   REPORT  45 

equal  compression.    The  ultimate  for  repeated  stresses  is  for  stresses 
of  the  same  sign  greater  than  the  elastic  limit,  for  stresses  of  oppo- 
site signs  smaller  than  the  elastic  limit.    It  should  be  remembered, 
however,  that  the  tests  made  by  Wohler  and  Bauschinger  were  made 
with   stresses   repeated   rapidly   in   immediate   succession,    while   the 
conditions  in  a  bridge   member   are  entirely   different. — The   permis- 
sible unit  stress  should  remain  therefore  for  the  ordinary  working 
load  below  the  ultimate  for  repeated  stresses,  while  for  extraordinary 
conditions  it  may  exceed  the  ultimate  for  repeated  stresses  without 
danger  to  the  structure" — 
in  other  words,  for   extraordinary  conditions,  occurring    only  a  few   times, 
ii  ever,  during  the  lifetime  of  the  bridge,  it  could  for  stresses  of  the  same 
sign  without  danger  even  exceed  the  elastic  limit. 

The  statement  concerning  our  ignorance  in  regard  to  the  design  of  com- 
pression members  has  to  be  taken  with  reservation.  Bridge  Engineering  is 
not  an  exact  science.  Any  engineer  who  prepares  a  design  and  follows  it  through 
the  drawing-room  and  shop  will  admit  that  there  are  a  hundred  and  one  ques- 
tions where  his  "string  of  formulae"  proves  insufficient  and  he  has  to  rely 
on  his  judgment,  consciously  and  unconsciously  derived  from  past  experi- 
ence. The  lacing  of  compression  members  is  a  detail  and,  like  other  details, 
can  only  partly  be  analyzed  theoretically.  The  connection  of  floor-beams 
to  the  posts,  particularly  of  floor-beams  cut  out  for  the  pins,  in  fact,  all  riveted 
connections,  including  splices,  especially  such  with  several  ribs  and  many 
rivets,  are  more  complex  matters  than  the  lacing  of  a  column.  But,  after 
all,  is  our  knowledge  of  tension  members  as  complete  as  pretended?  We 
have  tested  eye-bars  and  single  wires  or  small  ropes  in  full  size,  but  these 
are  not  tension  members,  merely  parts  of  a  tension  member,  and  about 
the  distribution  of  stress  over  the  cross-section  of  a  cable,  for  instance,  we 
have  only  approximate  ideas. 

Table  No.  5  gives  unit  stresses  of  those  main  members  of  the  trusses  Appended 
which,  according  to  the  reports  of  the  experts,  show  most  unfavorably.  The 
unit  stresses  due  to  the  dead  load  were  taken  not  from  the  stress  sheets  used 
in  the  design,  but,  to  avoid  criticism,  were  derived  from  the  dead  load  stresses 
of  the  experts,  reducing  the  stresses  by  an  amount  due  to  the  reduction 
of  the  paving,  pipes,  railings,  to  their  originally  assumed  weight.  Six  sets 
of  unit  stresses  due  to  three  different  conditions  of  the  "congested"  and 
of  the  "regular"  Uve  load  were  compiled  from  the  "Loading  Key"  and  com- 
bined with  the  stresses  from  dead  load,  as  follows: 

(1)  "Congested"  live  load  in  continuous  stretches; 

(2)  "Congested"  live  load  in  discontinuous  stretches; 

(3)  One-half  of  the  "congested"   live  load  over  the  whole  bridge 
(from  end  to  end)  and  one-half  in  discontinuous  stretches; 

(4)  "Regular"  live  load  in  continuous  stretches; 

(5)  "Regular"  live  load  in  discontinuous  stretches; 

(6)  One-half  of  "regular"  live  load  over  the  whole  bridge  (from  end 
to  end)  and  one-half  in  discontinuous  stretches. 


stress  Sheet 
Table  No.  5 


46  CHIEF   ENGINEER'S   REPORT 

Recommen-         Based  on  these  figures  and  the  foregoing  discussion  on  the  probability 
of  the  different  Uve  loads,  the  writer  recommends  the  following: 

(1)  The  paving  should  be  reduced  to  its  originally  intended  weight  on 
the  whole  bridge,  or,  which  may  be  even  more  effective  for  its  final  purpose 
of  reducing  certain  stresses,  reduce  the  paving  only  on  the  river  spans,  leav- 
ing on  the  Island  span  and  the  two  anchor  arms  the  heavy  paving,  as  the 
writer  suggested  to  you  about  three  months  ago,  before  the  reports  of  the 
experts  were  known.* 

(2)  With  the  paving  altered  in  this  way,  the  bridge  is  safe  for  the 
intended  traffic,  viz.:  two  promenades  of  11  feet  each,  a  roadway  of  35^ 
feet,  four  trolley  tracks  of  1,000  pounds  per  linear  foot,  and  four  elevated 
railroad  tracks  of  1,700  pounds  per  linear  foot. 

(3)  The  experts  state  that,  since  the  bridge  was  designed,  the  weight 
of  trolleys  and  rapid  transit  trains  increased,  the  former  from  1,000  to  1,460 
and  the  latter  from  1,700  to  1,810  pounds  per  linear  foot.  This  may  or  may 
not  be  only  a  passing  phase  in  the  development  of  the  rolling  stock  of  elec- 
tric traffic.  In  the  next  few  years,  before  any  rapid  transit  traffic  will  cross 
the  Blackwell's  Island  Bridge,  many  changes  in  the  rolling  stock  may  occur, 
the  cars  may  get  longer,  or  the  character  of  the  traffic  may  change  (moving 
seat  platforms,  etc.),  and  the  calculations  will  have  to  be  revised  for  the 
new  conditions. 

(4)  To  remove  the  stringers  designed  to  carry  two  elevated  railroad 
tracks  is  not  necessary,  and  not  advisable,  as  they  now  support  the  foot- 
walks  and  new  ones  would  have  to  be  provided  to  replace  them,  and,  as 
nobody  can  tell  whether  they  may  not  be  of  use  within  the  next  ten  years, 
should  the  weight  or  character  of  rapid  transit  traffic  change. 

(5)  A  thorough  investigation  of  the  actual  traffic  on  the  existing  bridges 
in  New  York  City  should  be  made  by  the  Engineering  staff  of  the  Depart- 
ment of  Bridges  (not  by  laymen)  in  order  to  prove  conclusively  that  the 
traffic  needs  no  police  regulations,  beyond  those  customary  in  the  case  of 
ordinary  city  streets  on  which  "congestion"  or  "bunching"  of  traffic  to  the 
extent  of  50  pounds  per  square  foot  over  any  great  area  would  not  be  toler- 
ated by  the  police,  and  to  establish  again  that  sense  of  proportion  which,  in 
this  whole  controversy,  seems  to  have  been  lost. 

In  taking  instantaneous  photographs  of  the  traffic  of  the  Williamsburg, 
the  Brooklyn  and  other  bridges,  possibly  also  on  crowded  streets  (from 
an  upper  story)  and  approximating  from  them  the  live  load  over  a  certain 
length,  a  fair  estimate  of  the  weight  per  square  foot  at  different  hours  and 
its  maximum  could  be  established,  while  the  usual  assumptions  of  50,  75 
or  100  pounds  per  square  foot,  derived  from  experiments  in  buildings  with 
stationary  live  loads,  are  for  floors  of  long  bridges  hardly  better  than  guess- 
work. Very  truly  yours, 

F.  C.  KuNZ,  Chief  Engineer. 

*See  Supplement,  page  47 


CHIEF   ENGINEER'S   REPORT  47 


Supplement  to  the  Report 

Steelton,  Pa.,  December  28th,  1908. 

Since  this  report  has  been  written,  the  Department  of  Bridges  has  pre- 
pared new  drawings  for  the  paving  of  the  roadway,  which  is  now  under 
construction.  The  change  in  the  paving  consists  in  a  reduction  of  weight 
on  the  river  spans  to  approximately  that  originally  assumed,  leaving  the 
heavy  paving  on  the  Island  and  anchor  spans. 

Table  No.  6  shows  the  stresses  of  the  truss  members,  based  on  the  final 
paving,  corresponding  to  table  No.  5,  based  on  the  original  paving. 

The  stresses  given  in  the  report  refer  to  the  original  paving  and  are  changed 
as  follows  for  the  final  paving: 

Stress  for  Stress  for 

Original  Paving  Final  Paving 

Page  2.5,  twelfth  hne  from  top   20,700  21,100 

Page  25,  twelfth  line  from  top   18,800  18,900 

Page  2.5,  thirteenth  line  from  top 34,100  34,200 

Page  2.5,  fifteenth  line  from  top 22,900  22,700 

Page  28,  sixth  line  from  bottom 21,200  21,000 

Page  28,  fifth  line  from  bottom 33,500  33,600 

Page  30,  ninth  line  from  top    18,600  18,400 

Page  30,  tenth  line  from  top 22.900  22,700 

Page  30,  eleventh  line  from  top 25,600  25,400 

Page  30,  twelfth  line  from  top   34,200  34.000 

Page  30,  sixteenth  line  from  top 20,.300  20,100 

Page  30,  twentieth  line  from  top  29,000  28,800 

Page  42,  seventeenth  line  from  top 22,100  22.200 

Page  42,  nineteenth  line  from  top 17,000  17.100 

Page  42,  twentieth  line  from  top     11,900  12,100 

F.  C.  K. 


APPENDIX  A 

EXTRACTS  FROM  REPORTS  OF  EXPERTS 

"The  provisions  of  the  specifications,  both  for  the  chemical  and  physical  Extracts  from 
requirements  of  the  material,  are  in  accordance  with  the  best  practice  of  ^e  j^epo^  of 
the  present  day,  and  entirely  satisfactory.   I  have  as  far  as  possible  examined  William  H. 
the  manner  of  inspection,  both  in  the  mill  and  shop,  and  I  have  scrutinized  ^^^ 
carefully   a  mass  of  records  of  experimental  data,  established  by  tests  of 
both  specimens  and  full-size  eye-bars  in  the  course  of  mill  inspection,  with 
satisfactory  results.   All  of  this  class  of  evidence  goes  to  show  that  the  material 
put  into  the  bridge  was  of  excellent  quality  and  fully  (met)  the  requirements 
of  the  specifications." 

"The  character  of  shop  work  is  evidenced  by  the  condition  of  the  manu- 
factured members  in  the  bridge.  These  are  largely  open  to  ocular  inspec- 
tion, and  I  have  many  times  been  on  the  structure  for  the  purpose  of  exam- 
ining the  results  of  the  shop  work.  I  believe  it  to  be  fully  up  to  the  require- 
ments of  the  specifications  and  generally  in  accordance  with  the  best  prac- 
tice of  the  present  time.  The  four  sub-diagonal  posts  C56-L57  and  L107- 
C108  on  both  trusses  near  the  main  piers  on  Blackwell's  Island  were  a  little 
twisted  when  put  in  place,  but  this  difficulty  was  corrected  by  riveting  cover 
plates  on  the  tops  of  the  posts.  These  posts  are  not  main  truss  members 
and  the  question  of  the  safety  or  stability  of  the  latter  is  in  no  way  affected 
by  them." 

"Many  sections  have  been  carefully  calipered  and  the  weights  of  many 
members  have  been  computed  in  order  to  determine  whether  the  actual 
dimensions  of  pieces  as  placed  in  the  structure  correspond  to  the  require- 
ments of  the  specifications,  contract  and  shop  plans.  The  results  of  these 
examinations  have  been  entirely  satisfactory.  The  actual  sections  of  the 
members  are  generally  found  a  little  full,  a  margin  of  2^  per  cent  being  allowed 
by  specifications  in  accordance  with  common  practice." 

"I  have  made  a  careful  examination  of  the  entire  structure  as  far  as 
possible,  with  a  view  to  determining  whether  evidence  of  distress  of  any 
members  exist,  such  as  permanent  distortion,  loose  rivets,  or  other  evidences 
of  overloading,  misfitting,  or  any  other  circumstances  which  might  indicate 
over-stressing.  I  have  not  found  such  evidence.  All  parts  of  the  structure 
appear  to  be  in  satisfactory  condition.  There  are  minor  or  small  variations 
from  aUgnment,  which  are  usually  observed  in  large,  completed  bridge 
work  in  place,  but  nothing  whatever  to  indicate  any  inherent  weakness 
or  unsatisfactory  condition." 

"In  addition  to  this  inspection,  I  have  had  accurate  surveys  made  of 
the  entire  structure  to  determine  the  alignment  of  trusses  and  elevation  of 

49 


50 


EXTRACTS   FROM   REPORTS    OF    EXPERTS 


lower  chord  points  in  July  and  as  late  as  October  26,  the  past  month.  These 
surveys  indicate  that  the  alignment  of  the  trusses  is  entirely  satisfactory, 
and  that  the  deflections  vertically  are  only  those  caused  by  the  dead  weight 
of  the  structure  at  the  two  dates  stated,  the  dead  weight  at  the  latter  date 
being  considerably  more  than  at  the  former  in  consequence  of  the  large 
amount  of  floor  material  put  in  plaCe  between  the  periods  named." 

"Adverse  comment  has  been  made  on  the  heavy  compression  lower 
chords  of  this  bridge  and  their  design  therefore  has  been  scrutinized  with 
great  care.  .  .  .  No  criticisms  of  these  lower  chord  compression  members 
would  probably  have  been  made  except  for  the  failure  of  chord  sections  of 
somewhat  similar  general  shape  of  section  in  the  Quebec  Bridge.  The  simi- 
larity, however,  lies  only  in  the  general  form  of  section  of  the  component 
parts.  The  spacing  details  of  the  Blackwell's  Island  chord  sections,  consist- 
ing of  heavy  lattice  bars,  batten  and  tie  plates,  and  transverse  diaphragms, 
are  relatively  far  heavier,  stiffer  and  stronger  than  corresponding  details 
in  the  Quebec  trusses;  indeed,  in  the  latter,  there  were  no  transverse  dia- 
phragms such  as  are  found  in  the  Blackwell's  Island  Bridge.  There  is,  there- 
fore, little  or  no  similarity  as  to  the  unit-carrying  capacity  of  the  compres- 
sion chord  members  in  the  two  bridges." 

Professor         "(Second)  Both  the  shop  and  mill  inspection  were  efficiently  performed. 
Conclusions  resulting  in  securing  excellent  quality  of  material   and  the  fabrication   of 
truss  members  of  good  quality  and  accurate  dimensions.  , 

"(Third)  The  various  members  of  the  structure  possess  the  full  sections 
required  by  the  unit  stresses  and  the  working  plans,  and  the  shipping  weights 
correspond  correctly  to  those  sections  as  well  as  to  the  computed  weights. 

"(Fourth)  The  erection  was  successfully  and  satisfactorily  performed, 
leaving  the  trusses  in  correct  alignment  and  elevation." 


Extracts  from  "We  have  examined  the  detailed  reports  of  the  mill  inspectors  on  this 
Messrs.  Boiler  ^^^aterial,  and  find  that  they  show  the  metal  fulfilled  the  above  specifications." 
.  &  Hodge  "In  addition  to  figuring  the  stresses  on  all  members,  we  have  had  a  large 
number  of  the  actual  bridge  members  measured  and  calipered  in  the  field, 
and  we  find  that  they  agree  with  the  sections  we  took  from  the  shop  draw- 
ings and  used  in  these  calculations,  which  sections  we  show  in  detail  on 
sheets  8  and  9." 

"We  have  also  computed  the  weight  of  a  number  of  members  from  the 
shop  drawings  and  find  that  such  weights  agree  with  the  shipping  weights 
on  the  invoices,  showing  that  the  scale  weights  used  for  the  dead  load  are 
correct." 

"We  have  not  carefully  examined  all  the  details  of  this  structure,  but 
we  have  checked  the  end  connections  of  such  members  as  are  most  heavily 
stressed,  and  find  them  equal  in  strength  to  those  members." 

"We  have  carefully  considered  the  form  and  details  of  the  lower  chord, 
as  this  feature  has  been  criticized  in  the  public  journals,  and  the  impression 


EXTRACTS  FROM  REPORTS  OF  EXPERTS 


51 


has  been  given  that  the  lower  chords  in  this  structure  are  weaker  than  those 
of  the  Quebec  Bridge,  which,  in  our  opinion,  is  not  the  case." 

"We  have  made  a  careful  examination  of  the  bridge  as  now  completed, 
and  find  no  evidence  of  loose  rivets  or  buckling  of  members,  or  other  indi- 
cations of  overloading,  but  there  are  four  sub-diagonal  posts,  C56-L57  on 
both  trusses,  and  L107-C108  on  both  trusses,  which  had  a  "wind"  in  them 
during  erection,  and  this  was  corrected  by  riveting  a  cover  plate  on  the 
top  of  each  post.  It  will  be  noticed  that  both  of  these  members  are  "sub- 
diagonals,"  which  in  no  way  affect  the  main  stresses  and  are  only  for  the 
support  of  one  local  panel  load,  and  the  stress  sheet  shows  that  they  will 
never  be  subject  to  heavy  stresses,  so  they  are  evidently  safe  and  the  "wind" 
was  probably  due  to  a  bend  in  the  shop  or  during  erection,  or  to  a  slight  over- 
run in  length." 


"(Second)  That  the  steel    manufactured   for  this  structure  is  first-class  Messrs. 
bridge  material  and  in  accordance  with  the  specifications.  Hodee' 

"(Third)  That  the  workmanship  of  this  structure  is  first-class  and   in  Conclusions 
accordance  with  the  requirements  of  the  specifications. 

"(Fourth)  That  the  erection  and  field  riveting  of  the  structure  appears 
to  have  been  done  in  a  first-class  manner. 

"(Fifth)  That  the  actual  sections  of  the  various  members  agree  with  the 
sections  ordered  on  the  working  drawings  and  shown  on  our  sheets  Nos. 
8  and  9,  and  that  the  shipping  weights  are  correct." 


APPENDIX  B 

EXTRACT   OF  SPECIFICATIONS   FOR  THE   STEEL  SUPERSTRUCTURE 
OF  THE  BLACKWELL'S  ISLAND  BRIDGE 


All  steel  shall  be  made  by  the  open-hearth  process,  and  shall  fulfil    the 


Manufacture 
and 
Requirements  following  requirements: 


CHEMICAL   REQUIREMENTS 


NICKEL  STEEL 

Phosphorus  P   C.  Max 

Sulphur 
P   C   Max 

Nicke, 
P   C   Min 

Basic 

Acid 

Eye-bars  and  Pins 

04 

06 

05 

3  25 

STRUCTURAL  STEEL 

Plate  Shapes  Bars  and  Pins 

.04 
04 
05 

.08 
04 
.08 

05 
04 
05 

Rivet  Steel           

Steel  Castings     

62 


m 

H 

!z; 

W 

S 

H 

P^ 

s 

I— ( 

00 

f. 

l:^ 

O* 

m 

K 

H 

iJ] 

« 

S 

W 

U 

M 

i-:i 

a. 

o 

<1 

'^ 

O 

h-l 

»3 

>H 

a 

(1^ 

H 

3 
w 

< 

O 

I— I 
m 


1     - 

^ 

0 

3T3       -So    ^i?. 

^5 

■*^  "f1 

O               2  O  .ti  (N 

gl 

•  -  ^  ^  i:  c  i- 
?2  ^  lP^a-2 

CQfa 

2 

■p 

00                C 

o 

I— (             >— 1 

' 

2  3 

<P        iH 

si 

— '  ^*    to 

^t^ 

u 

O    i^!    O 
"^^   CS   o 

s 

a 

0  c5 

"5  » 

3"< 

^ 

pepjooaj  aq  ojl 

■O^ 

<C  0 

Pi 

1    a; 

C    fc- 

—    3 

.-f^ 

i                c 

-if   " 

c 

9^  <s 

V   ;-. 

c3 

U 

GO   bC 

c 

^    C 

c 

W 

C^ 

•-    3 

1 

^■3 

C2                 1 

j 

J3 

^— ^^— ' 

o 

C 

i"      ■§ 

!? 

^g 

1     3 

m 

^ 

.2 

OJ 

^ 

a, 

(3 

K} 

w 

^ 

!            ^ 

s 

c 

3 
0 

X 

a. 

j= 

_2 

n 

c 

d 

gj3 

CT 

g 

a 

g 

Sx 

-c 

o 

c 

LO 

5 

0 

X 

CL, 

■ 

x: 

V 

ClJ 

0) 

c 

c 

cS 

>— ^ 

X                  w 

u                 u 

c8                es 

Xi               ^ 

j)                11) 

>>              >> 

fc 

K 

1 

o 
o  & 

X 


SOS 
cS 


13    3 

3-^ 


-t-3 


a- 
O 


3  O 

o  (P  ^  o  a) 


S3  S  i=! 


« 


>5 


O 

s 

o 


02 

a 

a 

c 

c 

^ 

j»i 

c 

t 

J3 

>5 


54 


EXTRACT   OF   SPECIFICATIONS  55 

Tensile  tests   of  structural   steel,   showing   an   ultimate  strength   within  Allowable 
4,000  pounds  of  that  desired  will  be  considered  satisfactory.  strength^ '° 


LOADS 

The  bridge  shall  be  proportioned  to  carry  in  addition  to  its  own  weight  Live  Loads 
and  that  of  the  floor  a    live  load  either  uniform  or  concentrated,  or  both 
as  specified  below,  placed  so  as  to  give  the  greatest  strain  in  each  part  of  the 
structure. 

For  the  main  members  of  the  trusses  and  the  towers: 

(a)  A  load  of  8,000  pounds  per  linear  foot  of  bridge  as  "regular,"  or 
(6)   16,000  pounds  per  linear  foot  of  bridge  as  "congested"  traffic. 
For  the  secondary  members  of  the  trusses,  the  floor-beams  and  the  floor 
system: 

(c)  On  each  elevated  railroad  track  a  load  of  52  tons  on  four  axles, 
6+10+6  feet  apart  (the  motor  ends  of  two  motor  cars  of  the  Inter- 
borough  Rapid  Transit  Co.). 

(d)  On  each  street-car  track,  either  a  load  of  26  tons  on  two  axles 
10  feet  apart,  or  a  load  of  1,800  pounds  per  linear  foot  of  track. 

(e)  On  any  part  of  the  roadway  a  load  of  24  tons  on  two  axles 
10  feet  apart  and  5  feet  gauge  (assumed  to  occupy  a  width  of  12  feet 
and  a  length  of  30  feet),  and  upon  the  remaining  portion  of  the  floor 
a  load  of  100  pounds  per  square  foot. 

(/)  On  the  footwalks  a  load  of  100  pounds  per  square  foot. 

The  wind  pressure  shall  be  assumed  as  a  moving  load  acting  in  either  ^ffiui^ 


direction  horizontally  Avith  2,000  pounds  per  linear  foot. 


PROPORTIONING    OF   PARTS 


Pressure 


For  the  main  sections  of  members  of  the  trusses  and  towers,  no  material   Least 

Thick] 
of  Steel 


shall  be  used  less  than  one-half  an  inch  thick;  all  other  material  shall  not       ^*^    ®^^ 


be  less  than  three-eighths  of  an  an  inch  thick. 

All  steel  work  shall  be  so  proportioned  that  the  maximum  strains  from  Permissible 
dead  load  and  live  load,  or  dead  load  and  wind,  shall  not  cause  greater  unit  ^°'*  strains 
strains  than   the  following: 


{a)~For  Nickel  Steel  in  Eye-bars  and  Pins — 

Tension 

Shear  on  pins 

Bearing  on  diameter  of  pins 

Bending  on  outer  fibre  of  pins 


(b)  For  Structural  Steel  in  Main  Members  of  Trusses, 
Towers  and  Bracing — 

Tension 

Compression 

Shear  on  shop  rivets,  bolts  and  pins 


Bearing  on  diameter  of  shop  rivets,  bolts  and  ] 
pins ) 

Bending  on  outer  fiber  of  pins 


For  Dead  Load  and 

Regular  Live  Load 

or  for  Dead  Load 

and  Wind 


For  Dead  Load  and 
Congested  Live  Load 


Pounds  per  square  inch 


30,000 
20,000 
40,000 
40,000 

20,000 

20,000-90-* 
r 

13,000 
25,000 
25,000 


39,000 
24,000 
48,000 
48,000 

24,000 

24,000-100-* 
r 

16,000 
30,000 
30,000 


(c)  For  Structural  Steel  in  Secondary  Members  of  Trusses — 

Tension  in  subverticals  (hangers) 

Compression  in  subdiagonals 

Shear  on  shop  rivets  and  bolts 

Bearing  on  diameter  of  shop  rivets  and  bolts 


(d)  For  Structural  Steel  in  Floor  System  of  Roadway  and  Footway 
and  in  All  Floor  Beams — 


Tension  chords 

Shear  on  shop  rivets,  bolts  and  web-plate  net  section . 
Bearings  on  shop  rivets  and  bolts 


(e)  For  Structural  Steel  in  Floor  System   {including  brackets)   for 
Railroad  and  Trolley  Tracks — 


Tension  chords 

Shear  on  shop  rivets,  bolts  and  web-plate  net  section 
Bearing  on  shop  rivets  and  bolts 


Pounds  per  square  inch 


18,000 

18,000-80-* 
'  r 

12,000 
24,000 

15,000 
10,000 
20,000 

10,000 

7.000 

14,000 


*  Where  1  =  length  and  r^  least  radius  of  gyration,  both  in  inches. 


56 


EXTRACT    OF   SPECIFICATIONS  57 

Members  subject  to  reversals  of  strain  shall   be  proportioned  for  each  Reversals  of 

kind  of  strain,  and  the  section  shall  be  determined  by  the  strain  requiring  ^*''*'° 
the   greater    net    area. 

For  combined  strains,  due  to  dead  load,  regular  live  load  and  wind,  the  Combined 

■unit  strains  given  above  may  be  increased  20  per  cent.  strains 


CONSTRUCTION 

Provision  shall  be  made  for  a  free  expansion  and  contraction  of  all  parts,  Temperature 
•corresponding  to  a  variation  in  temperature  of  110°  Fahrenheit. 

The  deflection  of  the  spans  from  dead  load  shall  be  taken  out  by  correct-  Camber 
ing  the  length  of  each  truss  member. 


Cross   Section 

BLACKWLLLS      ISLAND      BRIDGE. 

with 
Two  "Rapid.  Transit  RK.  TracKs 


58 


Cross    Section 

blackwe.ll:s    island    bridqe. 

wrth 

Four    Rapid    Transit  R.R.  TracKs 


59 


TABLE  1 

Diagram  of  "continuous"  Live 
Load  for  Chords 


TABLE  2 

Diagram  of  "continuous"  Live 
Load  for  Web  members 


TABLE  3 

Diagram  of  ""discontinuous"  Live 
Load  for  Chords 


lb 


TABLE  4 

Diagram  of  "discontinuous"  Live 
Load  for  Web  members 


TABLE  5 


Unit  Stresses  for  various  conditions 
of  loading  with  original  paving 


\'ty 


MANHATTAN  ANCHOR  ARM 

Ar. 

as 

::: 

■  j'i 

is 

I(^U.            1          ii,n,=.«d 

'^^. 

„^|N_. 

, 

Moic 
3U,0 

138.3 

1         ISLA 
+2M+2T.7+I6. 

-i: 

1    '-'^  I  Usii  iaa^a 

ANTILEVER    ARM.  E 

'  r,  :  1  1 
*M|il:S|lS,1|*qi'? 

11 

AST 

'i. 

V. 

MO.(lh  in.2 
-ia.o  -is,3 

1 

8 

MANHATTAN   CANTILEVER    ARM 

ISLAND 

CANTILEVER 

ARM    WEST 

.    _       1 

KM 

U..l«^ 

ToUklLfflid 

SSi  *S?t 

Af=i 

"m" 

Li.«  L«<l                                1                              Toul  L<Md 

P.miu«U> 

o™.    »., 

s:.St:s' 

..,=:,.. 

CbnioUd 

"*                                     lanm 

».i 

iK:-.rSl?iS 

.^:;:..'iir.j{J^|.,^:^.!:;r::tT!!i:' 

HAW  POSTS           i 
QUEENS    ANCHOR   ARM 

Rwilai 

as. 

4-24.0 

+3»!o 
+  30.0 

-20.0 

-aio 

% 

-ISO 
-10-R 

-aoo 

'SOS 

-isii 

+  30.0 
+30-0 

C30-L31 

108.1 

8SU 
30.0 

* 

Q 

UEENS    CANl 
lUIH 

i:ii.izii: 

OHORD 

-18.0 

1 

i 

.10.. 
-...0 

*Mi 

-1 

*i>>.l|i.29.<) 

rtU.?  ^27:3 
♦45.0'  ^24.0 

-4fi.ol  -lo.e 

Ii 

-14-0 

U  9S-U  ttT 

|83o!o 

+I5.0l  +  ^;s     11  1 

'::':': 

11:! 

i 

-It. 
-10.2 

1    Z!-'Z 

6ie.( 

408.0  -f2O.0 

i;:!i:; 

-  oil 

l7<!o  +H^ 

-  8.1 

ilii 

+18.0 

-10.11 

KAIH 

-  so; -  ^1  -  so 

-ao.3 -aj.B  -20,1 

-21 .41  -22  0  -10^-. 

-206  -a*  -auj 

« 

UIOS.LU 

s 

»~~ 

VE  aTRBasES 

*" 

IN    UNITS    OP    1.0 

ISLAND   SPAN 

._ 

IJn  Loa.1                               1 

IV,. 

To«l  l.o«.l 

o'lXl 

IbnC-M      _ 

>.„>.,         1 

las' 

ll»I. 

LOa-Lflf 

1 

388.1 

ii! 

S08.3 

\  itJt 

', 
' 

sir,  ikI  „'.::..!  sir 

';;  ■,:|;:;  ,, 

SI 

GREATEST  STRESSES   FOR    VARIOUS 

MEMBERS 

1 

QuwM 

iWul.i 

MuilMf    UtrM      II«nt*i      Slii 

-iTJlT',!!!! 

U>mU( 

i™.     .■.„„,     .,™     ».„w     ..«. 

"; 

im- 

Si  "-^1!^  l»;J".M"Lm-^'« 

1 

zi!' 

1 

3;E:ii 

o 


BLACKWELL'S  ISLAND  BRIDGE 


THE    PENNSYLVANIA  STEEL  CO. 


TABLE  6 

Unit  Stresses  for  various  conditions 
of  loading  with  final  paving 


MANHATTAN  ANCHOR  ARM 


MANHATTAN    CANTILEVER    ARM 


ISLAND  CANTILEVER   ARM.  EAST 


-9io!+20.ol 


ISLAND    CANTILEVER    ARM    WEST 


ISLAND  SPAN 


QUEENS  CANTILEVER  ARM 


QUEENS  ANCHOR  ARN 


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FOR   VARIOUS  MEMBERS 


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BLACKWELL'S  ISLAND  BRIDGE 


Mb*.  I    S^aMlbt. 


I  STRESSES  ARE  ( 


1^ 


TABLE  7 

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UC  SOUTHLRN  RtGIONAL  LIBRARY  I  A(  JL 


AA    000  915  347    9 


University  of  California 

SOUTHERN  REGIONAL  LIBRARY  FACILITY 

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LOS  ANGELES,  CALIFORNIA  90095-1388 

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